I'd love to see a Falcon 20, King Air 200, Lear 31, and maybe a Baron 58, all in 1/48.
For 1/144, an injected 737-200, Convair 240, EMB-120, DHC-8, DHC-7, Beech 1900C, and 717. Most of these are already available in vac and/or resin, but I really don't look forward to beating a Sasquatch Convair 240 into submission.
BTW, Welsh has just released a vac/resin C-46 in 1/144.
Ben