I'm really not one for crossovers. Universe continuity is really important to me.
However, I'm all for big all-in-one universe compilations like in Subspace Emissary. Then it's just silly fun and I don't have to think too hard about why such and such character exists in this other unrelated universe. Nintendo needs to do more stuff like that.
(Nintendo Kart, anyone?)
Bloodbath Paddy Wagon Ultra 9