Villager doesn't vibe with me, but I see no problem with them.
Mega Man desperately needs a facelift. He doesn't have to use his Mega Man 11 design, but for the love of fun let the kid have nuanced expressions and voice lines. His gameplay though, while difficult to wrap your head around, is absolutely perfect once you do. The only thing I'm kinda sad about is that he'll probably lose the ability to cancel Leaf Shield by grabbing/tossing items in the next game, which would make it useless again.
Wii Fit Trainer is a mess. I'm not sure they're unsalvageable, but they need focus. Given her projectiles, the fact that Deep Breathing wants you to go in, and the lack of body hitboxes making it hard for her to do so my guess is she's supposed to be a setup zoner of some kind. There's two major problems with that though: 1, Sun Salutation and Header don't set up anything, in fact, it's more likely for other factors to set up Header. 2, Sun Salutation and Header are so strong and her normals are so awkward to land a hit with that she ends up not ever wanting to go in without Deep Breathing.
I don't know if this would necessarily work, but here's my proposal for her: Header has been adjusted so it's easier to make the ball a lingering threat. Mainly, it doesn't just rocket off stage when hit with most attacks. Perhaps you could even make it launch toward Wii Fit Trainer when you hit it with Sun Salutation, which now also sets up combos at low charge. Deep Breathing has been reworked to a longer lasting, 3-tiered buff. Tier 1 ups damage, and gives certain moves slight disjoint, as the guide arrows from her exorcise demonstration videos appear, acting as smears and the like. Tier 2 causes her moves to pulse a colored sillouette when she strikes the full pose, giving her body hitboxes, and upping her damage a bit more. Tier 3 turns her into the absolute monster that it used to, and opponents will want to avoid allowing her to get this far.
Each tier of Deep Breathing takes more time to reach than the last (and using the move during tier 3 just heals her), so her gameplan will be to use her projectiles zone out opponents so she can use Deep Breathing, setting up combos to get her damage in in the meantime. Once in teir 1, she's able to play the midrange game a lot better, and in teir 2, she's able to actually scrap with opponents, though she'll still want to rely on Header and Sun Salutation to control space. She generally won't be allowed to reach teir 3, but if she is, she becomes a character to fear, with a ton of kill confirms.
Rosalina & Luma are good. I've seen people dissatisfied with how Luma is controlled, and maybe there needs to be more of an indicator of how far it goes or something, but it's not like the premise isn't solid.
Little Mac at the very least needs to be able to interact with platforms. He doesn't need to have a stellar air game, but he needs to be able to get up to platforms and challenge opponents that are trying to run away from him. Dunno the best way to do that, but it needs to happen.
Greninja is fine I guess. The skill floor is too high for me, but they're fun sometimes.
Mii Brawler is pretty good, though some of their specials need some rebalancing to make sure the others are good enough in comparison. Mii Gunner is pretty good, though she's a bit too evocative of Samus, and some of their moves are like...why? Mii Swordfighter has problems that throw the whole trio out of whack. I think their midrangeish projectile zoner playstyle is cool, and they do have the ability to play more aggressive as well, but the fact that they're a projectile zoner conflicts with the fact that that's supposed to be Mii Gunner's thing. That, and he's supposed to be the basic swordfighter character. There's something to be said about allowing the custom characters have diverse ways to play them, but in that case the other two need the ability to play in ways that break the mold of their archetype too, but also even in that case, there's no build for Mii Swordfighter that allows for them to play as a normal swordfighter and that should be rectified too.
Palutena is fine. I somewhat agree with the idea that she's not as interesting as she could be and she doesn't exhude the power she should as a goddess, but as a baseline, I think she's fine, and I don't really have any strong opinions about her.
Pac-Man is probably as he should be. Maybe he's a bit overtuned, but that's not really an inherent design problem. There are a few characters that can't deal with him at all, but I think that's more a them problem than a Pac-Man problem.
Robin has a good idea, and most people seem to be satisfied with the execution of it. I have seen a decent amount of complaints that he just kinda gets punished for playing the game by having his special moves be tied to a resource when they're not even especially powerful. I've also heard the argument that the management aspect of his gameplay is negated by optimization; he gains nothing for not being optimal, so being optimal is the only choice, and then you get punished for it. While I'm not sure I'd go that far, I do agree that durability has very little risk/reward going on, and could benefit from giving the special moves extra options that cost more durability. Have players ask if they want to use the more powerful tool now, or wait until it's going to have a lot more impact, kinda like how the risk/reward is managed with Thoron now.
Shulk needs some cleaning up, but for reasons I've stated earlier, I think the direction they took him in makes sense, and has the potential to be good, but it's still got a few kinks, and there is some missed potential with how Vision works.
Bowser Jr. isn't generic. His attacks are actually quite creative, and his special moves can be pretty interesting as well. He's just...his moveset doesn't come together. He's a heavy, but not really. He's got setups, but they're difficult to pull off. He has an item, but that's more of an afterthought. Everything in his kit is just kinda half baked. I don't even know what direction I'd take him in, but he's going to need a much more focused vision going forward.
Duck Hunt is perfect, except I don't really like the current implimentation of Clay Pigeon. Kind of a minor thing though.
Ryu feels clunky to me, and I think he'd benefit from being given the option to do strong attacks with the grab button (you could still grab by pressing attack + shield). You could frame it like a classic/modern controls thing. I would also like the option for all command input characters to have the option to disable command inputs from the attack button, requiring the special move button to be pressed, but I care about that less. Aside from that I'm not sure I have too many strong opinions about him or Ken.
No opinion on Cloud.
Corrin feels like he should be more interesting than he is. I do wonder if his pin shenanigans were too much in the previous game so the backpedaled too much, but I don't like that pin is only useful for hitting people on the ledge now.
Bayonetta is mostly fine. I kinda wish Witch Time slowed you down a bit more, but I understand why it doesn't.