100% Synchronize On Stationary Pokemon: I not only agree, but also would like to this be applied to Event Pokemon as well.
Bottle Caps To Change Hidden Power Type & IVs to 0: I agree on being able to change IVs to 0, though honestly Hidden Power you should be able to choose its type another easier way (I'd also let you change what Category it was so Physical Pokemon can use it then). However, I'm curious is maybe Bottle Caps could also have Ability Capsule unlock Hidden Abilities and be used to change a Pokemon's Natures.
Less Backtracking (Sinnoh Issue): I guess they could do what they did in ORAS by having a character letting you essentially teleport back with them to the next location you're suppose to be at, but that could feel like hand holding. Maybe including new locations we could explore in backtracking locations would mitigate this problem, sure you're backtracking but at least there's new stuff to check out and maybe Pokemon to catch/trainers to battle.
No HMs (Sinnoh Issue): I feel this and the previous issues build upon each other. HMs are a pain in Sinnoh as you pretty much need either to fill a third or half your team with HM slaves in the originals. Guess they could have Poke Rides, would have to include new ones/change how some obstacles worked but it could work. Still, would be a same to see HMs not in the game, I liked Gen V/VI's methodology with HMs: not mandatory to progress but used to explore more of the area. I have a ton of idea myself how HMs could be painlessly brought back BUT I think that'll be too much wishlisting on my part so let's just move on:
More Ice-types Early/Mid Game (Sinnoh Issue): Mt. Coronet would be a good place to stash a few early Ice-types like Snorunt, Bergmite, Swinub, and Sneasel.
More Fire-types In General (Sinnoh Issue): YES. I get it, Sinnoh is a cold region, but can't throw in a Fire-type in the greener locations or the mines (Oreburgh City has a COAL mine! How is there no Fire-types)?
Difficulty Setting: Would like and better done than in B2W2 (like make if available in both versions from the start); though not likely.
Tropical Island/Alolan Variant/Island Guardians Locations: Eh... well, first off, let's keep in mind Sinnoh Remakes won't happen at least until Gen VIII is out on the Switch. So, much like with the Gen VI Pokemon, when they do the "Legendary Hunt" for Gen VIII they'll include the Gen VII Legendary. However, I'm skeptical if they'll create their own location just for them. Whatever they do for the other Legendary Pokemon I imagine they'll also do for the Island Guardians. As for Alolan Pokemon, ugh, well for Sinnoh there's an easy roundabout for that: Backlot's Trophy Garden (the Alolan Variant evolutions are a bit more tricky, maybe use the Strange Souvenir like have the Pokemon be holding it when it evolves).
Battle Frontier: PLEASE! Don't make the same mistake as ORAS! Plus the Sinnoh Battle Frontier is mechanically more easier than the Hoenn one.
Unlockable Permanent Experience/Catch Rate Increase: That's still not going to help if there's no good grinding spots. We really need a White Treehollow/Black Tower location (which have trainers who use Blissey) to really have a place we can grind. Also the limit to use Hyper Training should be decreased to Level 50 or just something you can do once you become Champion.
Item Evolving Pokemon Very Likely To Hold Evo Item: Yeah, that only makes sense. Though I'd also just like to have one of those items available to get around the time you're starting to evolve your Pokemon (be it on the ground or given by someone) while extra can be bought via BP points. Also ALL the Evolution Stones should be available early to give the option whether to evolve those Pokemon early.
Eevee Available Early/Mid Game: Sure, some games have already done that too. Though the next issue needs addressing concerning Eeveelutions.
Location Based Evolutions Available Early/Mid Game: The Moss and Ice Rock shouldn't be hard to find an early location for, for Sinnoh specifically the Ice Rock is only a problem and, as I said with the Ice-type Pokemon, just place it somewhere in Mt. Coronet. However we can't forget the other locations. There's four we need to include: Moss Rock, Ice Rock, Magnetic Field, & High Location. Sinnoh only needs the last one and, once again, top of Mt. Coronet would be that location.
In-Game Trades With Trade Evolution Pokemon: Would be nice but won't happen, at least not without the Pokemon holding an Everstone.
"Demote" Another Mystical Pokemon (Sinnoh Issue): Now with Sinnoh having a whopping 4 Mythical, I can see them maybe demoting at least two. However for now let's focus on one: Manaphy. If they demote any I think Manaphy is a prime target, being it was originally given out not through an event but through another game. Unless they're releasing another Pokemon Ranger game for the Switch, might be better to have something like "Ranger Missions" (like Looker Missions) where you help Pokemon Rangers and in the end are trusted with a Manaphy Egg. As for the other Mythicals...
Release Event Items (Sinnoh Issue): I'd like to see them do the Event Items again, and this time releasing the Azure Flute as well (especially because the reason they didn't release it was STUPID). I'd also like to see them expand upon the events but that's another discussion entirely.
Version Exclusives: No opinions here though my list would be different, but I'm not getting into that.
Mesprit & Cresselia Don't Roam (Sinnoh Issue): Agree, though would like them to do something extra with them. For Mesprit maybe do what they did with the Winged Mirages in XY and have you chase them until following them to their nest. For Cresselia how about a side quest where several children around Sinnoh are afflicted by Darkrai and you need to cure all of them before Cresselia would battle you (you could also make this a requirement to use the Member Card, build up a story).
Golem Trio Are Available (Sinnoh Issue): Agree. Maybe require needing to see Regigigas first but if they want to keep the Regigigas event in they're going to have to include the Golem Trio in the game.
Giratina & Episode Platinum (Sinnoh Issue): Would be a shame to not to have the Distortion World in the game. Maybe at the climax of the main game story, they can have Giratina appear and snatch away Cyrus. Then we can have an "Episode Platinum" where, instead of Charon and the remainder of Team Galactic are trying to catch a Heatran, instead they're trying to figure out what took Cyrus and where he is. That'll lead into the Distortion World and we'll have the final encounter with Cyrus there plus get to catch Giratina, Cyrus than deciding to remain in the Distortion World and vanishing into the void.
Creation Trio & Ultra Space (Sinnoh Issue): Good point about using the Creation Trio (or maybe just Dialga and Palkia) to access Ultra Space. Hopefully, being Dialga and Palkia are gods, we can more control where we go (thus better prepare). I do not like the Ultra Space Ride and do not want it to return, I find it needlessly complicated and too luck reliant for the end purpose. Also, unless they add a Megalopolitan (someone from Ultra Megalopolis) I don't think we'd be visiting Ultra Megalopolis. For Poipole we'd probably go to its world. As for Silvally, that's tricky as it's not suppose to exist until Gladion frees his Type: Null and evolves it... oh who am I kidding, they're just going to ignore that. Eh, just give it its own world in the Ultra Space too, maybe hint you traveled to a future Pokemon world.
Title Defense: Ooh, that'll be fun! For Sinnoh we have the Stat Trainers too to fill out the potential challenger roster. I can also see Saturn being a potential challenger, being he sort of tries to create a new Team Galactic after Cyrus vanishes. Hmm, who else. Maybe Cynthia's little sister? Also, I highly doubt Dawn/Lucas will use the other version Legendary on their team.
Meloetta Forms: Hmm, using a Mega Rayquaza mechanic to change Meloetta between forms is one way to go about it. Though honestly I'd rather they just gave us a Key Item to change its form, like those Red Shoes we heard it once had, and be done with it. Also, have Relic Song change form along with Meloetta, like Aria Forme has Relic Song which is a Psychic-type Special move that has a chance of Sleep while Pirouette Forme has Relic Dance which is a Fighting-type Physical move that has a chance of Paralysis.
Luvdisc: There's nothing that'll help Luvdisc, not even a Mega Evolution (unless they change Mega Evolution mechanics). Might have potential with a gimmicky exclusive Z-Move. I'm picturing a Z-Move that creates a field effect that any Pokemon sent out becomes infatuated against Pokemon on the opposite side of the field that's the opposite gender. To really mess with the opponent, give Luvdisc a Signature Move that changes its gender to the opposite of its target. :P
Castform: Easy one. Just have Castform's Special Attack and Speed double (or even increase by 1.5x) when Forecast activates. Maybe even allow Forecast to summon a weather condition matching a weather summoning move that Castform knows (or make that into its own Hidden Ability).
Maractus: I'd say give it a Mega Evolution that makes it slow but very powerful and an Ability which makes it immune to Sandstorm like Sand Veil (mainly because how did this species survive being a pure Grass-type in locations always buffed by a sandstorm?). That should give it a niche in Trick Room, Sandstorm teams.
Farfetch'd: The problem with Farfetch'd is that it's suppose to be a bad Pokemon as a joke. Giving an evolution or even Mega Evolution would kind of go against that. That does leave a Signature Z-Move though. To go with it being a joke, how about it Z-Move has it cook a stew that fully heals itself and its ally while Badly Poisoning and Confusing the opponent's Pokemon. Don't know what the activation move would be, maybe have a way for Farfetch'd to learn Softboiled? As for the Stick item, how about instead of always hitting a Critical Hit it only does so with items with an increased Critical Hit ration (Slash, Night Slash, Sky Attack, Leaf Blade). And better Hidden Ability, how about Super Luck. ;)
Sudowoodo: Think a Mega Evolution would do, also give it No Guard.
Silvally: Eh, I think Multi-Attack is fine as is. The Memories though have potential. Being a disc, maybe they can affect Silvally in a way like have the RKS System act like another Ability which matches the type of the Memory.
Dunsparce: Yeah, give it a Mega Evolution. Make it part or pure Dragon-type.
Articuno: They're not about to redo a Pokemon's entire stat spread, and if they did Articuno would not be up first. I think if any were up first it would be the Eeveelutions...
Nerf Stealth Rock: I think capping the max amount of damage it does at 1/4th would suffice. Also make it break when it activates on a Rock or Steel-type.
Unown: Now I've had my fair share of ideas on how to improve Unown, though with Wishiwashi that does open up a new opportunities. Question is how would a Schooling Unown be different from Wishiwashi? And would it make be worth doing another idea?
Sunflora: Uhh, maybe a Mega Evolution if distributed right and gets to be part Fire-type? That said, I do want to mention here I always thought some of the Abilities which require sunny weather (Solar Power, Chlorophyll, Leaf Guard, & Flower Gift) should give the user a resistance to Fire-type moves just so Grass-types with those Abilities aren't making one of their weaknesses stronger.
Entei & Flareon: These two are very similar so they might as well get the same suggested moves. Extreme Speed, Wild Charge, and Outrage I can see. Not sure about Iron Head. And no to Head Smash as I don't think they have the proper biology (which is also why it wouldn't get the Ability Rock Head. Sheer Force still a possibility).
Solrock & Lunatone: THEY'RE A SUN AND A MOON! HOW DO THEY NOT LEARN MORNING SUN AND MOONLIGHT?!
Hypno: Hypno is gonna need more than bulk and Recover to it viable though I guess it doesn't hurt. As for Inner Focus, maybe it's not it needs replacing but being made better?
Swalot: More defense and a recovery move never hurts, though don't think much can help Swalot, not even a Mega Evolution.
Fearow: Odd, no reason it shouldn't had a secondary Ability. Eh, give it Gale Wings.
Chansey family: Odd again, the move seems to be tailor made for the Chansey family. Also Blissey needs a Mega Evolution so it could at the very least compete with its pre-evolution.
Seviper & Zangoose: Two other Pokemon that could get creative Mega Evolutions. I can see Seviper turning into a dragon and Zangoose into an steel clad knight.
Carnivine: I'm not really sure what can be done for Carnivine outside of a creative Mega Evolution.
Regigigas: Decreasing to 3 turns and it getting a temporary defense boost could work. One idea I had was the counter also decreasing for every attack it took and once Slow Start worn off it would be cured of all Status ailments and stats reset. Only Hidden Ability I could see it getting would be Truant, MAYBE Stall. As for preventing Slow Start from activating again, the problem there is the way the game currently is that can't be prevented. Only way I can see them getting around Slow Start restarting after wearing off is if they gave Regigigas two formes: Inactive Forme and Active Forme. Inactive Forme would be how Regigigas starts out, having Slow Start as its Ability (it'll looks just as it does now but the colored gems on its body would be black). When Slow Start wears off, it'll change into Active Forme getting a different Ability (maybe Clear Body like the rest of the Golem Trio) and appearance will look normal (aka the gems get their color back). Will still have the same stats, this is just to stop Slow Start from re-activating.
Phione: Tail Glow would be nice though not much can stop it from being a cheap Manaphy. Infact I thought as a funny idea it could have a Signature Z-Move where it turns into a Phione-colored Manaphy.
Unfezant: Poor Unfezant, if it came a generation later they may have done something fun with it. It's sort of asking to have different stats depending on the gender, males probably being more physical built while females faster. Honestly they look like Pokemon who could have a strange movepool, like a batch of elemental moves. Could be just me.
Turtonator: Why doesn't it get Aftermath or Flame Body as an Ability? I don't think it's that concerned with coverage, probably would like more utility, like Rapid Spin. Could probably get Morning Sun as a healing move. I think what Turtonator needs is a reason the opponent wants to trigger Shell Trap/take out Turtonator first. Like if your opponent doesn't attack Turtonator on the turn it set up, next turn it'll attack with a 100 Power move and if its a Double Battle it'll hit both opponents.
Ultra Beasts: I'm surprised none of the Ultra Beasts have a Signature Move, closest is Buzzwole and Pheromosa having Lunge. Another thing I was expecting was for the Ultra Beasts to all share a Signature Z-Move, Ultrium-Z, using the Ultra Wormholes they dart in and out of wormholes and attack all around you.
Black Kyurem: Ice Fang, Ice Hammer, Ice Punch, Ice Shard, Icicle Crash, Icicle Spear, Come on GF! And Glaciate should at least be 90 Power and have a chance to freeze.
Plusle & Minun: Honestly the only thing I think they could do with Plusle & Minun would be with Plus and Minus. Find a way to buff the Abilities though not in the way it'll make other Pokemon who can get the Abilities be over powered thus no reason not to use Plusle & Minun. Also giving them a Signature Move may not hurt (maybe even a shared Z-Move? I don't think a Mega Evolution would help).
Darmanitan: There does need to be a way to use Zen Mode Darmanitan from the start (and remain that way for the rest of the battle), maybe a held item?
Ultra Dialga/Palkia/Giartina/Arceus: And this all reminds me, what do they plan on doing with the Creation Trio + Arceus? Give them Mega Forms? Give them Signature Z-Moves? Both? What about Giratina who has two formes with different stat distributions?