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Ghost/Dragon is one I would fucking love to see. Could be some skeletal thing like those enemies in Castlevania, or something more spectral/wraith-like, idk.
Ground/Fairy would also be pretty cool; could use a gnome archetype and make something out of that.
But/Dragon is another one that has a lot of potential. But mons have always seemed to have some of the best and most memorable designs, followed pretty closely by dragon mons, so a meeting would give you a lot of really awesome artistic possibilities