A26, bang for the buck. Something for bomber Jockeys would be cool, but anything new is always good.
I wouldn't call that "Bang for the Buck" as that would still require a whole new 3D model and flight model.
F4F-3's performance was different enough from the F4F-4 (being lighter, faster, and more maneuverable) yet would require only minor changes to the 3D model.
The clipped wing A6M3 Model 32. Once again, minor 3D model changes while giving different enough performance that it would basically be a new aircraft.
So that alone is two aircraft that could be dropped in relatively quickly with minor changes to assets already in the game.