1. Manga characters will not join the battle (obviously).
While it’s true that we’ve added characters from other companies,
It’s not going to be anyone and everyone.
2. Smash is a game that’s made from everyone’s effort.
Both companies have to want the character to join the battle.
I need them to let me take charge of the character.
In order to fit the character into the world of Smash, I need them to allow me some artistic license.
3. The character must carry game history with them.
Besides that, it might be something like a courtesy to include a character who has the experience of being on a Nintendo platform.
At any rate, there will probably only be one or two more. –Toukouken #3
Source:
https://www.sourcegaming.info/2015/10/05/sakuraithirdparty/