Ridley isn't that far fetched. He'd probably need to be shrunk a bit, but even Bowser was large enough to swallow Mario in one gulp back in the SM64 days. Of course they could have him use a very crouched stance like Faust from Guilty Gear (my favorite fighting game character ever), but then it might become problematic if he's too long.
Metroid is a major franchise with no other suitable characters, and a heavy flyer with ranged attacks would be incredibly awesome, so I think they'll at least try to get him to work.