I know that PMDT has stopped working on future patches but I'm still going to express my opinions, and show my wish list:
I think Olimar is a quite solid character, but he suffers a lot in some specific areas. Mostly recovery, but also tech-rolls, and some pikmin related stuff are quite annoying.
Ionium Jet: make it so you have more angles, sort of like Pit's up-b in Smash 4, you can go completely vertical and then go almost all the way horizonal. Also make it have a little less end-lag, Fox only have like 7 frames, and he has a way better recovery and also a better neutral than Olimar, wtf!?
Airial Pikmin Pluck: gives greater horizontal boost, which would aid recovery.
Whistle:
-Make it have slightly more end lag, so you can't spam it, BUT add some heavy-armor. This could add an option getting down from the air and as a recovery, which are two of Olimars weaknesses. So you could mix up tanking an attack with whistle, but if the opponent is smart he can bait it out. The heavy armor also gets weaker when you reach higher percent, as with every move that has heavy-armor.
-Make it switch pikmins quicker. It's quite slow actually.
Roll and Tech Roll: make them slightly better.
Grabs: add grab armor, why does every character except Olimar have that!?
Pikmins:
-Make it so pikmins doesn't just fall when Olimar goes into tumble. It's really annoying to have to whistle them back again.
-When grabbing the ledge, make it so that the pikmin first in line is the one you do your actions on the ledge with. As of now, if I want to do a B-air from the ledge to edgeguard, I have no clue which pikmin I'm doing the B-air with. It seems like depending how low you drop from the ledge, different pikmins becomes "first in line", really confusing.
RNG: Maybe alter the RNG slightly. For example, if you haven't plucked a yellow, let's say, in the last 10 plucks, the chance will increase slightly until you pick a yellow, and if you haven't plucked one in 20 plucks it will increase even more. This will make drawing pikmins of a desired color easier, so you wont pluck like 40 pikmins, none being a purple. However this will make getting more of the same pikmin more difficult.