Hot take: I respect wanting to change a veteran’s Final Smash (because GOD KNOWS some of ‘em need it), but it legit makes me CRINGE when people suggest Final Smashes that only exist to make a character look cool.
For example, Peach and Daisy don’t NEED their super moves from Strikers Charged; just change the latter’s FS to inflict a flower effect on all fighters. Plus, giving Mario the T-Rex is an okay idea, but timing and development issues probably would’ve caused complications.
For those who DO need it, though? Hoo, boy, where to start? Well, expanding on Mario for JUST a bit, they should’ve at least given HIM a Mega Mushroom, and kept it for Ultimate.
TL;DR I’m okay with FS change, just as long as it stays true to that character.