I appreciate Sunshine a lot, not just because it was a really really fun game, but because it did something fresh with Mario. Fludd was great. And - perhaps I am in the minority of this - putting the red plumber in a tropical island instead of the usual Mushroom Kingdom helped to expand the Marioverse me thinks. New sights.
And the Galaxy games did the same thing in this regard with space. So really, a big part of me is wondering where could Mario's next grand adventure be, you know? It's a relatively minor thing to fixate on, although a 64/Sunshine open world could really make this pop.
I liked Sunshine a lot better in this regard. I thought it was great how focused Sunshine was on developing the island culture. They didn't give it as much depth as, say, one of the locals in a given Mario RPG, but the place had a lot of character.
What would really be awesome would be if Nintendo used a main series Mario game to explore some of the places created in the various spinoff games. Like what if we had a Mario 3D platformer centered around Rogueport? Or the Beanbean Kingdom? It would let Nintendo capitalize on nostalgia and let them interact with the rich character some of these locations already have.