Realistically, I have no idea what Nintendo would do, as they've been quiet about Wario land for a little over 10 years now. :/ But I think about what it could be sometimes, even though I can never fully idealize one single idea. Here's the most solid one I have (not solid, just moreso than my other ideas):
Maybe we take a tiny bit of the WarioWare canon and make it about Wario living in the city and wanting a castle again, so he goes out to find one only to realize he has to steal it from its owners, who turn out to be actually evil forces. (Why not just build a new one instead of stealing it? It's Wario!) This is going by the timeline where Wario loses his castle at the end of Wario World, though, I'm not sure how that story would work out otherwise. Maybe he just wants a new castle...
Also, while I love the "get to the goal and go back through the level" thing introduced in Wario Land 4, I think they should try to find a new mechanic either to replace it or be a more primary factor than the timer in WL4 and Shake It. Stretching out of the norms of platformer has always been the draw of Wario Land, so trying new things would definitely benefit a new release.
I'd also love to see Captain Syrup as an straightforward antagonist again.