The problem is, how would they really affect the gameplay? The mechanics in Brawl was that they could attack and even send the opponents flying. In Battle Royale, they already said there are Supers are the only way to kill. So Assist Trophies would only attack and not kill. Sure they can provide AP, but the boxes from Rachet and Clank do this already.
So I don't think they will be in it. Because there isn't a need for them. Their purpose in Brawl is redundant here. (This is my opinion though)