Kirby's fine, honestly.
He just needs specials that don't suck (well, besides the one that literally does) because everything else has a lot of strong stuff. And even that could be fixed with some rebalancing rather than completely changing them.
Most people would commit crimes IRL for their main to get a down tilt half as good as his. He's got solid aerials too.
Kirby just needs overall better Specials indeed. But that counts as a revamp already. I mean, take into account how much Ganondorf changed from 4 to Ultimate. Most people only really want the Specials changed now. I don't think it's realistic to ask for much more. Maybe a few more sword based normals. Down and Up Tilt come to mind but I doubt they're gonna change many aerials. Maybe Neutral Air.
For Mario, most just want FLUDD gone and maybe have him have Dr.Mario's D Air. That's what I'd do at least. But it's enough to call a revamp.
For Kirby specifically, I think we need his normals buffed to be Brawl / Smash 64 level. He can be a combo centric character without being overbearing. Hammer as F Smash frees up a Special. Up B should be something better. Then maybe just maybe Side B can be Cutter as a projectile ? And with another imput Kirby could do the Final Cutter ? Stone as a D Air is another option however Kirby's D Air was always an important attack, if it was somewhat useful. So I dunno. Kirby needs a little Special rework and buffs mostly. I also would like if Copy worked a little more like it does in Kirby. Be able to inhale projectiles and what not and spit them back. It deserves more love.
Next up I think I want Zelda to have a few Tri Rod moves. And actual light magic in her normals. Change her Down B into Echo Summon. Charge up for stronger Echoes and keep it more or less similar to the Phantom Slash. She needs to do the zoner thing better.
Same with Samus. She needs to do the zoner thing better to. Make her Charge Shot and Missles work like in Metroid. It's her central skill skit. Anything should work around these moves. Her Grapple Beam throws, normals, you name it. All should resolve around her blasting you in the face. Also Super Missles. Charge up Super Missiles. For the love of everything holy. That.
Now Mewtwo. Confusion is cute, but I want real telekinesis. I want him to be able to land grabs and throws from a distance and mess everything up. Down B Disable doesn't do it. Everything should feel as powerful as Shadow Ball, F Air, and his Smash attacks. Go overboard. It's Mewtwo. He should be a very technical character that rewards the time invested. The insane combo potential is there already, but go a little further. More risk, more reward. But not too much glass canon, a little more beef here and there. More armor. This is Mewtwo. His presence should ooze power and intimidation. Not fragility.
So yeah basically character should feel more like themselves more or less.