Speaking of AT's, there's a ton that would be awesome to be playable this time around. Albeit, I don't use the term "promote" cause it makes it sound like the old role is gone. That won't happen at all(nor be replaced by anyone else) cause it's a massive waste of development time. For example, Springtron is a pointless AT addition. Why bother doing all that for no real benefit? You disable Spring Man when he's playable. Boom, done. If you even need to do that. Springtron also makes for a way better costume. Now, in the next game, that's a cool idea.
Also, Waluigi would be a godsend. Among the Super Mario characters, there's only three major characters(in terms of how big they are, not entirely referring to iconicness) are missing; Toad, Waluigi, and Paper Mario. I am not including Captain Toad on here because he's nowhere near as big as they are. Yet. He's easily another one soon enough, and by next Smash he'll feel like a weird omission if he isn't there.
There's others I'd still love otherwise; Bomberman is dead, but it sucks he's only somewhat there as playable. Isaac, Ashley, Takamaru, Lyn, Black Knight, Sukapon, Skull Kid(note I can't list Tingle here anymore, but him too), and that's just a few. Though honestly, Waluigi is the biggest one among this list(and I could see that at E3 too for the sole factor he would take the place by storm. There's others regardless who would too, like Crash or Master Chief.