I like the idea of a Mooney too. Always was a favorite of mine in full scale. Another rarely modeled civilian plane would be the Luscombe. Can you imagine one of those in a metallic covering? Hmmm....this is fun. Other planes I never see modeled (except maybe by scratch-builders) are:

Civilian: Grumman Tiger
Legend (kitplane)
Ercoupe
TBM 700
Pilatus PC-12
Mitsubishi MU-2 (OK that would be a tough one to fly I'm guessing)
Spirit of St. Louis (a nice big one I mean...not some park flyer)
Lockheed Vega (or Orion)
Northrop Alpha

Military: C-46
Brewster Buffalo
KI-61 Tony (agree with Bunstead on that one)
Any of the GulfHawk planes (the ones I'm thinking of were originally miltary designs, right?)
Oooo! Oooo! That Lockheed Q-Star would be cool (either version)

OK....I'm deep into "wishful thinking mode" now.