There are so many planes it'd be nice to have in the game, but obviously each time one is introduced, it's best if it helps gameplay, balance, or just starts to fill in one of the larger gaps... so my votes go for:
1. SM79 - good all-rounder, and give the RA a strike plane of its own
2. Pe2 - essentially the VVS equivalent of the Ju88 (and can we have fighter versions of both, please?!)
3. Do217E - to beef up the LW strike capability. I'd actually prefer the He111 first, but I doubt it'd be well received by many.
4. Wellington - standard Britsh medium bomber, and tough as nails with it!
5. FW200 Condor for long-range maritime patrol
6. Catalina- for Allied long-range maritime patrol
7. Rata
8. Fiat CR42
9. Gladiator
10. Hs123
-the latter four to give some interesting early-war options, and also for close support ground attack. I can imagine the Shvak's on the Rata might become popular in some quarters, despite its being an early war plane!