Who do you want i SSB but know that they will most likely never put in or really likely but you know. Here is mine:
Steve/Alex, Herobrine, Creeper or something else from Minecraft: You are probably most likely with the computer version of Minecraft but it actually started out as an XBOX game that Notch made to promote his workman ship and if they can put Animal crossing Villager in anything is possible.
Batman, Catwoman, Harley Quin from the Arkham Game series: I know that they'll never put this in because Batman is his own character but technical issues is not for this blog-post.
Sora, Riku, maybe Xehenhort: if I misspelt the names sorry but it would be awesome and Disney only owns 50% of the rights to the Kingdom hearts series so you never know.
American McGees Alice: NO! Just no!
Epic Mickey and maybe Oswald: I know for a fact that they will never put Mickey Mouse and Oswald the lucky Rabbit in unless Nintendo makes an agreement with Disney but I really love the first game and don't hate the second plus both characters have very unique gameplay so why not?
God of War: I don't know these games I just heard that they're good so I put it on the list.
Rayman: Like god of war I don't know him outside of the Rabbids series so yeah.
Rabbids: They're the Freaking RABBIDS!
Space Invaders or Galaga; I'm pretty sure that the owners of PacMan also own these games so it seems legit.
LEGO Indiana Jones and Flappy Bird: would suck
GTA: There are several protagonists in this series and Nintendo chose Little Mac out of several other boxers so picking one shouldn't be that hard
StreetFighter: Same Thing
Waluigi:meh
Please leave you ideas under comments and keep doing what you do