I'd love to see a story mode return, tbh. Subspace Emissary got mixed reviews, but if you don't like the story mode you don't have to play it.
As for characters, I'd love to see Snake come back. One Pokemon per generation sounds awesome as well. I want Generation 2 to have Pichu from Pokemon Trainer's team, Generation 3 to have Deoxys (I think one character for all of Deoxys's Formes would suffice), Generation 4 to have Lucario, Generation 5 to have Zoroark (or any other Unova Pokemon tbh - I'm not picky), Generation 6 to have Greninja, and Generation 7 to have Mimikyu.
I hope Lucina stays on the roster, albeit with something to differentiate herself from Marth. I feel like Nintendo should keep the sole female representative of the Fire Emblem franchise. I'm also hoping for a return of Pokemon Trainer, but with Pichu, Squirtle, and Ivysaur instead of Squirtle, Ivysaur, and Charizard because Charizard will likely be on his own. I know it's kind of a stretch, but I want Pichu back as a better fighter.
Yep, I'm still figuring out how to deal with it and how much to tell other people. I get sensory overload super easily, I have pretty bad social anxiety when talking to people in real life, and I get stuck on ideas sometimes. My dad tells me all that is because of my Asperger's.