It could be interesting to see more chars from the xenoblade series, especially if they can adapt their unique systems to fit the game like they did with the monado arts I'd have liked to see Dunban or Fiora, but we see both during Shulk's final smash, so ... Yeah, Rex and Pyra are next on the list.
I guess there'll be a pokémon to represent the last gen, too. Incineroar and Decidueye could be interesting, maybe Zeraora and Kommo-o too.
As for mains, I'll probably stick with Snake and Shulk, maybe some others like Link, Samus and Corrin. Dunno about the Inkling, I'm not convinced by what I've seen of it. I could be wrong tho.