The new ProBuilder helo rescue set is something I'm really looking forward to as I generally like helicopters. A set I would be very interested in in the COD line would be a UH-1 Iroquois (or at least a very close look-alike), somewhere in size between the "not-Blackhawk" and the "Urban Chopper". It would fit the theme of the Vietnam-era COD Black Ops sets that have been release thus far. It could also be designed with a more modern color scheme based on the UH-1Y Venom. For non military theme helo sets I think a news helicopter, mountain/winter search and rescue, police, and ski jumper set would be interesting ProBuilder sets.
An official Osprey and/or Chinook, or an approximation of, would be a set I would be interested in. Other non-military themed helos in the ProBuilder line would also attract my hobby spending money. There's an extensive variety of helo models that could serve as inspirations for sets form the Sikorsky Skycrane to the MH-53 Pave Low.