I've said my piece about not thinking a Koopalings deal for Erdrick would be worth the trouble, but I sure wouldn't complain if that's what we got.
In terms of representing other games from the series... well, you could take a stage from something that's not Dragon Quest III, though that might be set in stone depending on who you believe. I like the idea of Nadiria from IV/V with a bunch of bosses from across the series showing up, but then Dracula's Castle is already sort of that, so maybe not. There's also music, spirits (if they come with the DLC) and/or an assist trophy (haha, never). Of course, I think Slimes -> King Slime would probably be the ideal DQ assist trophy, but someone like Yangus or Veronica to stand in for a different game would work too.
My embarrassing dream scenario is some kind of "it was just japes, we really do care" Square Enix-themed update with more songs and an AT for Final Fantasy, an AT for Dragon Quest, and then an AT for Chrono Trigger to tie a bow on it. Robo or Ayla or something. Beyond never happening.