Most needed IMO:
Italian bomber, preffibly early-mid war.
Cr.42, largest produced italian fighter of ww2 & Will give us an early war italian plane for early med sets, 202 & 205 fill Mid - Late war pretty well.
Gloster Gladiator, Opponent for Cr.42 and would make some great med scenarios/tods/ct setups!
I-153 & I-16 (Must have I-153 with I-16, HTC is not allowed to model 1 without the other!!!! I-153 was actuelly superior to the I-16 in many ways, and out produced the I-16 during ww2)
He111 & B25 - both very important ww2 bombers.
2nd most important:
Ki43/44/45 to fill out japanese early-mid war plane set.
PZL (cant remember rest of the name) but would be very cool addition

Ki-84, not really needed for late war planeset IMO since the A6m2,ki61 and N1k2 are definitly able to compete with allied planes, but too many people want it not to include it.
Medium Russian bomber, il-2 is great ground attacker, but they need something a little heavier!
3rd most important:
variants of planes we allready have, like early war P-38. not the most important, because we can substitute easily now, especially with new skin editor, but its much harder to substitute planes that don't exist yet, look at midway and TBM sustitution for kate!