Now I'm not saying these a/c are vital, or that they balance any issues. Just that they would make interesting additions.
Messerschmidt Me410 : Interesting either as a JaBo or a Pulk Zerstorer: many wepaon loadouts, funky canopy/cockpit, weird defensive guns.
Mitsubishi J2M3 Raiden: I don't think any sim has modelled this one. A Japanese Zoom and Boom specialist, anyone?
Polykarpov I-16: Make a nice little base defense plane in the MA. Armament isn't bad. Pocket-sized russian fighter.