Remember that ORAS will shuffle around level ups. BW2 did that with BW movepools to make mons that had really stupid levelups viable.
Some notes:
Zigzagoon / Linoone: Nice job skipping Linoone, jerk >:(. Play Rough is neat. Could be cool if GF decides to throw the poor thing a bone and give it earlier Belly Drum. Imagine Espeed by level @_@
Surskit / Masquerain: Personal bias here, but I think that Masquerain is probably viable this gen. You now get Bubblebeam relatively early and actually have STAB that's not physical. Scald is a learnable tm. I think you're underestimating how great Quiver Dance is; it makes literally any Pokemon an ingame god. I imagine that Air Slash will also probably get to a lower point in its level up (same goes for Bug Buzz, I imagine). Early Struggle Bug would be great.
Slakoth / Vigoroth / Slaking: You can abuse the switch prompt with Return / Giga Impact if you kill something (not unlikely given 102 or 150 BP coming from 160 base attack), so they're actually pretty good if you can get past Slakoth. Vigoroth can hold evio if you want entertainment.
Mawile: How good this is is determined entirely by when you get its mega stone. Even with Iron Head / Play Rough, it's still really mediocre. It doesn't get any strong moves or STABs until Fallarbor. Getting this from Granite Cave makes it garbage; getting it from Origin Cave or Victory Road is probably ok since I imagine you get its mega stone by then and also even have the ability to farm heart scales from the luvdisc that appear nowhere but before victory road.
Plusle: This actually seems not that bad. Electro Ball substitutes for Shock Wave kinda (ignores double team but has the potential to hit harder I guess) and Encore is actually pretty cool. Nasty Plot is particularly useful for it; I could see it have nice synergy with Encore, too. How good this is depends on when it gets GK / Thunderbolt. All of this goes for Minun too, but it's weaker than Plusle for negligible gain in bulk.
Volbeat: As terrible as this looks, Tail Glow makes it decently powerful. At +3, it hits kinda hard. Shame it can't BP the boosts, but something like Signal Beam -> Bug Buzz / Tbolt / Tail Glow / filler could work.
Cacnea / Cacturne: I'm betting this thing will get its levelup fixed. Why in hell is Needle Arm at 53; that's just hilarious. Any early STAB move and early Sucker Punch especially will make it really cool. Swords Dance is also potentially great.
Kecleon: If this gets hordes, it'll be cool. Protean is annoying as fuck ingame but having STAB on everything is neat.
Shuppet / Banette: A new mega and Knock Off are neat. Gets Shadow Sneak and Sucker Punch too.
Duskull / Dusclops / Dusknoir: This is honestly one of the few mons where I have no clue what the fuck GF was thinking. Dusclops makes sense as a wall in gen 3, ok. Dusknoir, for whatever reason, is a Pokemon without STAB in a generation that literally gave STAB to almost every single Pokemon that needed it. This is not usable ingame unless you desperately want to troll around with 60 BP from 100 base attack. It's awful, lol. Hilariously, best I can see is evio dusclops with Night Shade.
Tropius: At least it gets Sweet Scent that's useful now :). Leaf Blade in level up would be cool. A gimmicky idea I can see is Growth + Sunny Day (a town and a route away from Tropius) + STAB moves + Chloro.
Clamperl: Maybe Clamperl gets Shell Smash earlier now? Maybe we get DeepSeaTooth easier now? One can only hope.
Other Hoenn Dex mons:
Abra / Kadabra / Alakazam: This thing loses Shock Wave and special Thief but that doesn't stop it from being very good.
Goldeen / Seaking: FUCK YEAH SEAKING Mediocre but it now has physical Waterfall, Megahorn and Poison Jab. Neato.
Magikarp / Gyarados: Physical / Special split makes it able to sweep anything it chooses. Guarantees victory against every elite four with DD + Waterfall + Ice Fang + Filler. Mega stone will make it even more broken.
Geodude / Graveler / Golem: Better attack. Gets Rock Blast as a Geodude very early on and gives it a non-ass STAB rock move for the rest of the game. Probably still a very solid ingame contender.
Zubat / Golbat / Crobat: I kinda wish we had a mega crobat now. Gets Acrobatics and the inferior-to-Sludge-Bomb Cross Poison. Acrobatics is neat but it does not get anything good that is not Flying-type. Still, better than it was in RSE, I guess.
Tentacool / Tentacruel: A solid ingame contender as always. STAB Bubblebeam -> Surf will always be enough for it. Sludge Bomb / Wave help whenever you get them.
Machop / Machoke / Machamp: 100% Submission is ok STAB. 100% Rock Tomb is neat.
Magnemite / Magneton / Magnezone: Great Pokemon that gets more broken when you evolve it. Will be very strong ingame. Whatever TM Wattson gives, it's gonna be good for Magnezone.
Voltorb / Electrode: Ass. It can't hit a Pokemon with an Electric resist without Explosion or the godly Rollout. Still awful. Come on, give it surf or something; it's dense enough to act as a buoy...
Oddish / Gloom / Vileplume or Bellossom: Oddish starts with special acid now, which is pleasant. Vileplume is "how long do you want to delay Gloom's evolution" and Bellossom is "I caved on Vileplume because I wanted STAB above 40 BP). How good it will be is dependent on when it gets STAB TM moves.
Doduo / Dodrio: Bird loses its physical Tri Attack. At least there's nothing competing for the Return TM anymore, I guess. Stronger Fly.
Slugma / Magcargo: Worse because it loses Flamethrower for Lava Plume. I never realized how bad this thing is ingame lol; it does not evolve until LEVEL 38. Awful and sad because Magcargo is cool. Even gets Shell Smash (don't laugh; with 100 Speed EVs from Vitamins, a 15 IV and a neutral nature, it's going to outspeed somewhere around base 90s with a neutral nature and 15 IV.. not bad imo) on evolution and Ancientpower / Earth Power with move relearner.
Grimer / Muk: Why this doesn't have Poison Jab in its levelup is beyond me. Poison Jab TM placement changes how good it is.
Sandshrew: Magnitude alone makes this so much better. Slash + Magnitude is good enough until you get Return / maybe Dig. Swords Dance is cute and you have random TM moves now.
Skarmory: Awful 'til you get your slightly stronger Fly and probably decent after that. Over the gens, this thing has become a lot less impenetrable ingame. I remember it feeling really mediocre in BW2.
Igglybuff / Jigglypuff / Wigglytuff: Absolute garbage unless it gets something like Moonblast or Boomburst as level. Worse Exploud that you get later in the game. When you evolve it, it learns nothing. Dazzling Gleam TM placement determines whether it even has a shot at being nonshit.
Staryu / Starmie: Might as well evolve it the second you get it. You have Surf by this point and it learns nothing by level bar Power Gem. Surf is good enough for it and Thunderbolt / Ice Beam / Psychic are all great. Same mon but still good.
Vulpix / Ninetales: Energy Ball + NP is cool. The second you get it, if you have enough Heart Scales, you can tutor it Flamethower + NP and get Energy Ball by TM. Maybe wait for Extrasensory; Calm Mind TM exists if you're too lazy to farm heart scales / there aren't any before victory road.
Pichu / Pikachu / Raichu: Catch a slave Nincada and use it to hunt for Light Balls. Grass Knot gives Pikachu more coverage and Volt Switch helps it not die on the first hit. Raichu is faster!!!
Psyduck / Golduck: Still mediocre in a game with so many better Water-types.
Natu / Xatu: Still bad. 120 BP Future Sight!!!
Girafarig: You'll have good Normal STAB to teach it by then. Probably ok but not really all that much better.... I guess there's no hard decisions on dumping Return on it.
Phanpy / Donphan: Magnitude by relearner, Assurance, Rock moves, Poison Jab, freer Return TM. Not waiting for a Ground move for the whole game...
Pinsir: X-Scissor + Storm Throw + Dig + Swords Dance = not bad. Mega = horrendously broken.
Heracross: Brick Break + Close Combat + Pin Missile -> Megahorn + filler is solid. Mega is very powerful.
Rhyhorn / Rhydon / Rhyperior: Bulldoze is nice. It gets Hammer Arm after evolving.
Corsola: Still garbage. Hustle is funny but still very bad.
Chinchou / Lanturn: Gets Discharge if you want it as well as Signal Beam.
Horsea / Seadra / Kingdra: "How many Heart Scales do you have for Dpulse?" or "How quickly can I farm max happiness for Dmeteor?" Not fundamentally bad before due to surf, at least.