Currently we've had either Cractus or another member of its species appear in WarioWare: Move It, so Nintendo and Intelligent Systems aren't against the idea of characters from one subseries appearing in the other.
But who else could make an appearance there?
Personally I feel Captain Syrup or Rudy could potentially appear as WarioWare characters in some capacity or another (albeit with a much more comedic role than in their original games), but who knows. Given Cractus appeared, theoretically any Wario Land enemy or boss could make a random comeback in a WarioWare title if it fit the story.
But who else could make an appearance there?
Personally I feel Captain Syrup or Rudy could potentially appear as WarioWare characters in some capacity or another (albeit with a much more comedic role than in their original games), but who knows. Given Cractus appeared, theoretically any Wario Land enemy or boss could make a random comeback in a WarioWare title if it fit the story.