Rosalina
A cosmetic change I'd like to see is to have the color of your Luma depend on your alternate costume, to avoid confusion. Also, a possible change would be to have Launch Star depend on Luma, which turns into the slingshot like in Galaxy / Galaxy 2. This gives more of an impact to losing Luma, since Rosa's recovery is significantly gimped without Luma – her Up Special could be replaced by a short-ranged hover or something until it respawns. And this goes without saying, but things like Luma's insane priority on attacks and wonky knockback values should be changed.
Koopalings
It'd be a good change to remove the silly Abandon Ship gimps that the opponent can pull on him. Aside from that, the main thing here would probably just be buffs and tweaks in various areas, such as his grab and killing options.
Greninja
Don't have a lot of ideas for this guy. Maybe Shadow Sneak's charging period could allow standard attacks / aerials to be used during it, to combo into the teleport itself? That'd be fun. Should probably make the teleport itself a bit more punishable at a distance, too, to compensate. Also, its ability to cancel hitstun should be removed.
Robin
A bit of added mobility, or reach on Robin's grab, would be nice. I'd also like to see one of Nosferatu's customs get the default spot, the one where the cloud is created a distance away. We don't have a ranged command-grab in the game as I recall, and it'd be interesting combined with the Thunder projectiles – make the foe shield at a distance in anticipation of a projectile, only to bypass it with a ranged grab. Oh, and the dash attack could be changed to be similar to Link's, in reference to an animation from Awakening.
Lucina
I could see Lucina getting the Ganon treatment – keep the same moves, but give them a unique flair. Like how her Shield Breaker animation is different, more inexperienced- and almost reckless-looking. This could be extended to the rest of her moveset, such as giving her more distance on each step of Dancing Blade (an aggressive play that might get her into danger), the command-dash custom option for Shield Breaker, Crescent Slash on Up Special, an altered Counter, and similarly tweaked standard attacks.
Corrin
Corrin's moveset is pretty alright overall. It could be neat to see Counter Surge replaced with some sort of full-on dragon transformation; maybe it'd be something that you have to work toward with a meter of some sort? Also, Dragon Lunge's non-kick options being made useful could be nice.
Palutena
The main thing here would be to give most of her attacks more utility, through general buffs. Her default special set should also be changed; maybe Explosive Flame, Super Speed, Jump Glide, and Lightweight would work.
Dark Pit
Like Lucina, Pittoo could get the Ganon treatment. I see him as more of an aggressive Pit, combined with aspects of Brawl Pit. So some of his moves become single-hit, combo-oriented moves, he becomes a bit faster but lighter, and his Up Special (Infinite Power of Flight), Side Special, and maybe Down Special could be reverted back to Pit's Brawl moves.
Villager
This guy's uair and dair need to change, RNG isn't constructive on a move like this. Maybe the moves could become stronger if you use them less often / weaker if you spam them? Aside from that, Balloon Trip's balloons could serve to be a bit more gimpable, as the move's intended "gimmick" is a rare occurrence.
Little Mac
Mac is really tricky in terms of design, since he's the swingiest character in the game; high highs and low lows. Maybe his recovery weakness could be helped a bit by making Straight Lunge's charge storable / making it a bit better distance-wise in midair? So you can recover if you're prepared and can get past the opponent's first edgeguard attempt, but you'll still need skill to make it back alive on your single Straight Lunge charge.
Wii Fit Trainer
I'm not even sure where to begin with her, she is meant to be somewhat of a joke character. I'd probably just give her better combo potential if you do manage to land her awkward hitboxes, and maybe a bit of an easier time killing?
Monado Boy
As for Shulk, the main thing he needs is slightly better frame data across the board. As for new ideas, one that I had was that Back Slash be replaced with Monado Purge, an energy projectile that has different properties depending on your current Art. Jump gives you a tall wall of a projectile, Speed is a quick beam, Shield is a barrier that blocks anything that comes at it, Buster is a multi-hit projectile, and Smash is an explosive projectile with big kill power. Maybe this could be on a recharge / only usable once per Art use? It'd also be nice if Vision were altered by your Arts, such as Speed making it a dash forward that's more combo-oriented or Jump making it a leaping slash that sends you both into the air for a follow-up. For that matter, Air Slash's second hit could go further forward while Speed is active.
Duck Hunt
The main thing here is to just make his smashes actually usable, such as removing Forward Smash's random heights.
Megaman
Flame Sword should have increased knockback, first off. It'd also be nice to have some kind of way to use Flame Wall like how it's actually used in the games (similar to Mii Gunner's sspec custom); maybe as a new Down Special?
Pacman
Mainly just fix his grab, honestly. Maybe give him a tad better combo potential?
Ryu
Ryu's pretty fine as he is.
Cloud
Cloud's also alright, I'd say. Could probably do with a couple nerfs to his onstage game, and maybe charging Limit could be more of a commitment?
Bayonetta
Finally, the Umbra Witch herself. I could see a few changes being made to her. Aside from patching some potentially broken techniques / toning down Witch Time / etc, I could also see her recovery in regards to Witch Twist being made more intuitive. For example, the double-jump "glitch" (?), where you jump and then immediately Witch Twist to keep your jump but refresh WT, could be removed. Bat Within and Bullet Arts could also bear to be more relevant; maybe Bat Within could have less endlag than a normal dodge and allow a counterattack with perfect timing, while Bullet Arts could be used to pressure shields by adding a bit of shieldstun / shield damage...? Bullet Arts in general are kind of awkward to work with, though.