In terms of stages I'd like, well, I want a Poké Floats 2. That would be pretty cool. I'd like a Pokémon Stadium with a form for every single Pokémon type as well, but that's never gonna happen. Oh, and also, Fourside plz.
As far as other character ideas go, one of them was vetoed when they removed transforming characters. The idea would be to have Hilda or Hilbert (yay for alt costumes) with Snivy, Dewott, and Emboar as their team. Their Final Smash would have them send out either Zekrom or Reshiram, with the Down Special Pokémon Change swapping between the two.
Also, it would be cool to have Pichu back with an updated moveset that gives him something to actually work with. I don't even care if they keep the self-damage, so long as he ends up with an absurd rushdown game to make up for it. (It doesn't matter if he damages himself if he never gives his foe a chance to hit him, after all.)
I already said it would be cool to have Isaac or Matthew in Smash 4, and if one of the two did make it in, then it would be cool to have Djinn show up as items. They'd be sort of like the Dragoon or Daybreak parts in that they don't show up one-at-a-time. What happens is, you go around and grab them, trying to get specific combinations to unleash a powerful summon when you press the Z button. You'd have the standard Venus, Mars, Jupiter, Mercury, as well as Ramses, Cybele, Judgment, Kirin, Tiamat, Meteor, Atalanta, Procne, Thor, Nereid, Neptune, and Boreas. Those are all the single-element summons. Then you get into things like Zagan (1 Earth Djinni, 1 Fire Djinni), Megaera (1 Fire, 1 Wind), Moloch (1 Wind, 2 Water), Flora (1 Earth, 2 Wind), Ulysses (2 Fire, 2 Water), Haures (3 Earth, 2 Fire), Eclipse (3 Wind, 2 Water), Crystallux (3 Earth, 2 Water), etc.