Rosalina has already gotten a fair share of buffs throughout the Ultimate patches, I'd say she's fairly decent by now; even if she may not be a top-tier contender by any stretch, she's functional in her own way. Doctor Mario as well is also in a decent position, I get the impression they're going to be risk-averse to buffing the character because the only thing that holds him back from really eclipsing his vanilla iteration would be his abysmal disadvantage, considering everything regarding his neutral and advantage state right now is pretty slept on.
Piranha Plant, K. Rool and Jigglypuff may not be buffed much simply because right now the designers seem a little too set in maintaining specific archetypes in their design. Little Mac and Bayonetta are most likely being kept on a leash because their tools run the risk of breaking the metagame if left unchecked.
Right now I feel like only two characters may either be eyed for - or at least require - imminent buffs.
The character that most needs a buff in general based on sheer ineffectiveness is
. I had high hopes for her early on when she was revealed but it's become increasingly clear that her basic toolkit is not only inferior in utility to the fighter she's based on (who's already not very good), she lacks kill power, has a mediocre disadvantage, and her neutral was overall stymied by the projectile nerf in previous patches. Having to revert a buff to her Fishing Rod was also completely unnecessary when there's very little else she has going for her.
The character that most needs buffs in terms of fixing things that just straight up don't work properly is
. Sorry Thinkaman and every other Sonic detractor on here, but after having a patch that largely fixed a bunch of characters' multi-hit moves - including Ken who was the most meme'd on character about this stuff, Bayonetta who most people thought never would be fixed, and Diddy Kong who basically jumped up a whole tier - there is no reason why Sonic is still donkey butt in being able to perform the few things that's even expected of him when his shortcomings have been known of since day one. It's kind of maddening given that the last patch seemed like it'd be perfect for him, and yet, Up Air and Spin Charge
still haven't been fixed, and there's even an undocumented change in the patch notes about how the Homing Attack now behaves. I read a theory somewhere that the fact that this change wasn't documented may imply that Sonic is possibly due for a major patch in the near future - perhaps they're just not entirely sure about his moves - but given the neglect this character has gotten in Smash overall, I'm not hedging my bets.