I have to admit, I'm in all for a massive Nintendo RPG. No, really.
The gameplay should be a mix between Fire Emblem (tactical RPG) and Pokémon (Moves execution, learning, forgetting, etc.).
The plot could involve some kind of evil force messing the multiverse and timelines (like in HW) forcing groups of Nintendo characters to fight together.
While it would have the obvious choices of characters like Mario and Link, I would love to see some rather obscure characters, like Sukapon or Captain Rainbow.