Mario
It's a toss up between Toad and Geno. I used to think it'd be Toad with no hesitation, but now I think I'd rather have Geno since he's kinda the poster boy for story based Mario, and I think we need story based Mario ro get a lot more attention and fanfare in this day and age where even the Mario RPGs are losing their story.
Donkey Kong Country
Dixie. Anyone else just feels wrong.
Yoshi
To be honeset Yoshi doesn't need anyone else but if I had to pick? Give me Kamek.
The Legend of Zelda
Give me Pig Ganon in the ALBW style with the trident. If we can have three Link clones, we can have two Ganons, where one Ganon is nothing like Ganondorf... it'd be the first proper representation of the series main antagonist to be sure.
Metroid
I honestly think Ridley and Samus are all we need. But if I picked a new one... I guess Sylux since he's looking to be the next big bad... but behing honest, I don't know much else about the character.
Pokemon
If I could have any character, I'd have another trainer. Maybe Lance, or N, or Giovanni, or Professor Oak, I don't want another protagonist.
Kirby
I don't think Kirby needs another one, but if I had to pick, maybe Marx or Adeline and Ribbon.
Star Fox
Krystal. If I can't have deconfirmed characters though, I'll take Slippy, though I'd hate to get Slippy without Peppy...
F-ZERO
Samurai Goroh for sure should be the next F-Zero character but again, if I can't have him, then Black Shadow.
EarthBound
Porky Minch for sure, but Masked Man is a good secondary choice.
Fire Emblem
I don't like Fire Emblem even a little bit but if I had to have another, can it please not be a swordsman? I don't know much aobut FE, but I do know they've got way more than swordsman.
Kid Icarus
Series antagonist Medusa is the only choice imo. I think she should have gotten in before Palutena if i'm being honest.
Wario Ware
Give Me 9-volt. I want him to use Nintendo toys like the ultra hand, chiritori, Labo, the Hanafuda cards... but also some original stuff like his hoverboard.
Sonic the Hedgehog
Dr. Robotnik. If you ask me, he deserves his spot in the series more than Tails does. Unlike a lot of villains, he's actually extremely active in every game he appears in.
...And everyone else I don't think I could pick a character that I would want.
I also don't want to do a retro pick, because I think it's weird and dumb to classify "retro" as one thing. That feels like classifying Arms, Splatoon, Animal Crossing, Wii Fit, and Pikmin under a "modern rep" umbrella term.