Nintendo was okay with making Daisy playable in Wonder, so it looks like they are easing up on the idea of letting certain characters appear in the platformers.
Mona is the one I most want playable. Maybe Captain Syrup can be an unlockable...
First thought is making that Crazy Galaxy April Fools thing real.
There's also Pyoro. Even though his games are unlockable in WarioWare, they ported his first game separately to the DSi shop as "Bird & Beans." So maybe a Pyoro collection.
I vaguely recall Nintendo saying that different 'versions' of characters are just the same character. If it's true of King Boo in Luigi's Mansion and the Mario spinoffs, then it makes sense it's true for Wario as well.