I-16
Yak-7A
LaGG-3
Yak-1B
La-5
P-39
Yak-9D
Imo the above would be a bare minimum when trying to fill out the gaps in just the fighters category for the VVS. However the number of VVS fighters represented in the game is frankly, meager and puny. Two Yaks and two Las, and one ground attack aircraft.
* The I-16 is often an 'iconic' aircraft which symbolizes the early VVS, just like the Zero is 'iconic' in representing the Imperial Japanese.
* The Yak-7A was originally started as a trainer aircraft, which came to be reconfigured and produced as a fighter on demand , due to the shortage of Yak-1s. IMO it was one of the first VVS fighters to stand any kind of real chance against the more advanced LW fighters, so it should be in.
* The LaGG-3 also stands side-by-side with the I-16s as the 'cannon fodders' and yet real 'workhorses' of the early VVS, and IMO having no LaGG-3s in an early Eastern Front setting is like having no Wildcats or P-40s in an early PTO setting.
* The Yak-1B, La-5, and the P-39 all represent the shifting phase of the VVS when it started to stand up against the LW, between '42 and '43. The Yak-1B is a truly excellent fighter on par with the 109Fs, and were among the best the USSR could produce in 1942. The La-5 is an important intermediate stage where the plane was a significant improvement over its predecessors (the LaGGs), but still lacking (until its ultimate transformation with the coming of the La-5FN).
* The P-39 is a must-have(while it would be categorized as a US plane) - it is THE plane where Soviet aces and legends were born, utilized to its utmost capacity in the VVS, numerous Soviet heroes like Rechkalov and Pokryshkin was born in this plane, and the Soviet elite "Guards" were also flying the P-39s.
* The Yak-9D would represent the beginning of the Yak-9 series, which along with the La-5FN in '43, became the bane of the Luftwaffe, and was destined to ultimately evolve into the Yak-9U. Also a very important plane.
.... and when the above planes become available, is when a few more 'treats' like the Mig-3 or the Yak-3 would be good.