Like half of the assist trophies.
Not half of them, but there's indeed quite a few characters there who would be worthy additions.
Lyn for example, is easily the most popular Fire Emblem character not yet playable. Only Ike has her beat in popularity.
Isaac is a very obvious choice I won't even need to defend. Just check his moves as a Assist Trophy, and imagine Earth element based powers amongst this, coupled with a potential Djinn system and the fact he is also a potent swordsman. Was upset he wasn't made playable.
Krystal has been a fan favorite for a long time, yet she's not appearing in modern Star Fox games so that would make things difficult.
Waluigi has the popularity, but would require to have a original moveset and I doubt that's considered to be worth the effort, as he doesn't have a potential unique gimmick or anything else interessting that he could utalise. So am doubtful of him still, unless he becomes a sort of crazy Luigi semi-clone of ... Luigi? Am not seeing it untill it happens.
Black Knight is also a favorite of mine, and would be happy to see him playable. Don't see it happening unless the Tellius games of Fire Emblem get a remake (which they should really consider anyway).
Also there's Skull Kid, and was also upset he wasn't made playable.
Outside of that, Dixie Kong had no reasons to be excluded, and Monster Hunter is kinda weirdly absent because of Rathalos. Outside of that, I wanted Impa real badly as well.