I think it's a too early to show off new characters, but I'm definitely down to see new fighters. Actually, that's a good way to put it: fighters. I hope to see a massive dump of Echo Fighters. That, again, is likely not up happen, at least not in the "massive" sense, because I bet Sakurai is still very shaky on the idea of Echo Fighters, and he'll be super modest about who can make the cut. He could essentially double or triple the roster with these things, but he won't.
And yeah, I'd love to see a return of Adventure Mode, but I and it to be longer (naturally, to accommodate for every character and franchise). I want it to have save spots so you don't have to blast through it in one go, despite the Switch's Sleep Mode. This way, I can go through about 30% of it and not have to start from the beginning later if one of you fuckers want me to kick your ass online.
Echo fighters are a great way to make money off DLC. Especially if their movesets are basically similar