There's obvious changes to be made, like giving him his voice and making him able to properly emote (his eyes tilt downward when frustrated or angry, upward when disappointed or worried, etc). There's also a lot more subtle things you could do to his animations to make him less of a bumbling astronaut and more... Olimar. For example, his current idle animations would be perfect to have on Louie since they're better fitting for him, but on Olimar they don't really convey much of anything. I'd make his idle animations convey more of his personality, like him looking ahead for potential dangers for one, and him doing some stretches for the other. A lot of animations could be made more distinct from one another like f-smash from side-b from grab, which are all currently very similar; at this point, changing them would be a QoL feature.
Give him this victory animation too, instead of the absolute handful of nothing his current victory animations are.
In my opinion they should have a complete overhaul of his moveset (right now it incentivizes the killing of Pikmin / using them as meat shields instead of their preservation which is just.... so backwards to what Pikmin is as a concept), but if we had to build off Ultimate then these changes are the absolute least I would want.
I have a whole moveset rework for him drafted, I just need to make art for it. Easier said than done, but it'll be more than worth it in the end becuase I'll know that at the very least I did my part.