The Mustang I was indeed fast on the deck.....by 1943/1943 standards. It'd be about average in the MA speed-wise--not super fast, but still no slouch.
"I think that if we ever get the early Mustang (or maybe Mustang I, I can't remember) that had 4 Hispanos, the P-51D usage would drop to about what the P-51B use is now. "
Then that my friend is reason enough to add it
I always liked flying the P-51D better in that "other" game where only a relatively few people used it.
A spitfire LF.IX would see a lot of use....10 MPH faster anf 500 FPM more climbrate at normal altitudes than the current Spit 9.....yeah, it's see so much use it might end up being a cheap perk like the C-hog (the unfortunate result of the British having such awesome fighters). Possibly not though, so it'd be a worthy addition either way.
The Yak-3 would see some use in either of its forms, it'd likely be used by players who like the performance of the Spit 9/N1K2 but want something.....unique. The earlier 1300 HP Yak-3 would probably be the best addition, for from a scenario/CT perspective and in terms of MA balance. The major limiting factors of the Yak-3 are lack of range and tiny ammo load.
Bf-109K-4: If this fighter is added, one would hope it'd be an earlier model K-4 (426 MPH) to represent the earlier batch of upgraded 109's Germany starting using (we already have a 450 MPH 109). The K-4 would likely become the dominant 109 in the MA since it'd handle better at high speed than the current choices.
The Ki-84, however, is the BEST choice of any of them. It's a Japanese fighter and would do much to balance CT setups and it also has a lot of utility for events. It has enough firepower and performance that people would fly it, WITHOUT it becomming too dominating (assumming that it is modeled using Japanese data). In addition, it's not am American or British fighter and hence would add more variety to the "color" of the MA.
Add the Ki-84!
J_A_B