Affection actually has some merit as a mechanic as long as you're facing the correct opponents.
It's easy to read complaints about the Affection bonuses and think people don't want them in, but we're not asking that. As you said, on a casual run it feels awesome when the Pokemon you stuck with through a lot of your journey dodges or hangs on with 1 HP from an otherwise KO hit (and/or scores a Critical Hit when they attack).
What most players looking for a challenge want is simply the option to go into the menu, select "Options", and right under selecting Text Speed there being something that says "Affection Bonuses On/Off" (along with "Exp. Share On/Off"). That's it, that's all we want. It can be turned on be default, just that little option. Actually it would be great if we can customize them even more like choosing what bonuses we want on and off (and how we want the Exp. Share to function, like it only affecting the Pokemon with the lowest level/isn't evolved), but baby steps.
1) I really like Roaming Pokemon.
I think the main issue with roaming Pokemon breaks down to two annoyances: Tracking & fleeing anytime.
Tracking: Even if you could see where they are on a map, they made Tracking them annoying by making it so you can't simply just Fly to where they are near. No, you gotta generally camp at a Route Gate which connects 3 or 4 Routes, running back and forth until the Roamer finally decides to teleport itself to that area. Infact "Tracking" really isn't the best word as you're not tracking it, and it's not "Trapping" nor "Baiting", it's just... "Waiting".
How To Make It Better: Okay, if they want to make it a bit challenging by not letting us use Fly, how about this: As long as the player is "on foot", the Roamer can't teleport. It's stuck travelling along the Routes it's on, occasionally going back-and-forth. That way, even if it takes some time, the player can reliably "track it" without worrying if they take too long it'll just teleport to a new location. Also, after an encounter they can have it teleport to a new location to start the tracking all over again, but once again as long as it can't teleport while you're on foot it's not that major of an issue.
Fleeing Anytime: Pretty much what the title says, even if its alseep, stats are all in the negative, and has low HP it can still run away. Even if it keeps the status ailment and HP it's still annoying.
How To Make It Better: Well first off if its asleep or frozen it can't run away until it wakes up (though you can have it have a higher chance of waking up early). Second have factors which decrease the chance of it fleeing (and have them stack): Low HP, Paralyzed, Confused, and lowered Speed & Evasion stats; those would be the most logical status ailments which would hamper it fleeing. How I see it, "fleeing" should be a "punishment" for doing poorly or not doing the most optimal strategy.
One thing I liked about Tierno was the concept of a themed trainer where it's not a type-based theme. They could have done better, made his team actually threatening, but the idea of "his entire team knows moves with 'dance' in the name and tend to set up" is a good way of both making him distinct and also making his various party members make sense. Oleana is the only other time I can recall having that happen.
There have been other instances but you gotta look for them. Ones that come to mind are:
Falkner: While a Flying-type specialist, most of his Pokemon are birds. I'm surprised they never tried doing a bonus battle where he surprises us by using a Blaziken and Empoleon. "What, they're bird Pokemon, nothing says they need to be Flying-type".
Lusamine: Oleana actually sort of copied Lusamine's shtick. Lusamine uses feminine and "cute" Pokemon.
But you're right we could use more of them and there's plenty of ideas, though whether they'll be an important character or just a side NPC with a neat gimmick if you notice it is another issue entirely.
The thing with Tierno is... he appeared one generation too early.
Was he? Sure, he couldn't make use of Oricorio or its Dancer Ability, but he could still have made use of moves like Dragon Dance & Quiver Dance to try and make a buffing up team (also throwing in some Petal Dance, Sword Dance, and Teeter Dance).
Did the Galar Tournaments allows improvement for their team? Looking at them individually… Not really. I’ll say it quick, they don’t even use a full party of six and are just barely better, still no using items even in the post-game.
Quick, give them to ILCA! Even if they don't make them good they'll at least make them interesting (I can imagine them giving Raihan a team for each weather condition).
Having a whole "friend group" for the journey was definitely a good idea in concept and it's clear it was a very bold and unique idea for X and Y, problem was the concept wasn't quite used to its full potential.
Checked a few posts ahead just to see if someone said this before I did. Well, not the whole "friend group concept" thing, though I agree with it. I just wanted to say I don't really have a problem with Tierno or Trevor (or Shauna) as characters but they're under utilized.
ScraftyIsTheBest pretty much summarized why so I'll leave it at that.
Actually, one thing I will add is maybe ways they could have improved it. Since they're pretty much travelling with us the entire journey, maybe they could have had them act as quick "side quests" for each Town/City we visit. Like Tierno would want to see (a) certain Move(s), Trevor would want to see certain Pokemon, Shauna would want to go with a player to see a certain landmark, and Serena/Calem would always like a battle. Also on occasion they'll switch up what they usually do or want to have a battle. You don't have to do these side quests, but doing them would give you more chances to interact with them and get to know them better, allow them to tell a bit more about themselves and their backstory cause, remember, you're new to the region and this group of friends so you wouldn't know much about them.
Melony might wish for an Ice-type Pseudo-Legend!
We absolutely need one of these someday.
You could pretty much say that for any of the neglected Types. Normal-type, Poison-type, Bug-type, Grass-type. Though we also need to do another thing: STOP MAKING THEM DRAGONS! There's mythical beasts of great power than just dragons, like focus on the Fairy-type or maybe do some Elementals/Primoradial beings (maybe even some folk heroes).