Bowser did get some pretty big changes in Smash 4 too. His moveset wasn't completely change, but the fact that he's faster and the new N-air and B-air makes him feels way more mobile and is imo way more fun to play. I think a lot of characters in the roster would benefit a lot for simpler change like that. Characters like Samus doesn't really need the whole moveset change, just change some basic attacks. I would change at least her jab, f-tilt, down smash and maybe down special (or at least makes it stronger).
Personally, I think the only characters that would need really big changes would be Ganondorf, Donkey kong, Toon Link and Sonic (and maybe Dark pit and Dark Samus so they're more different like Ken). There's other characters I would change, but it would mostly be small changes (like Mario, Samus, Kirby, Luigi, Dr. Mario). I think aside from the clones none of the characters from Smash 4 or Ultimate really need changes.