Most of the other aircraft in AH have 3+ variants, about time for the first Mustang to be avalible.
I don't think that is true.
Spitfire: 7 (counting Seafire)
Bf109: 6
Fw190: 5 (counting Ta152)
F4U: 5
P-47: 5
Hurricane: 3
P-38: 3
A6M: 2
A-20: 2
B-25: 2
Bf110: 2
C.2: 2
F4F: 2 (counting FM2)
La: 2
P-39: 2
P-40: 2
P-51: 2
Yak: 2
Ar234: 1
B6N: 1
B-17: 1
B-24: 1
B-26: 1
C-47: 1
D3A: 1
F6F: 1
Ju87: 1
Ju88: 1
Ki-61: 1
Ki-67: 1
Ki-84: 1
I-16: 1
Il-2: 1
Lancaster: 1
Me163: 1
Me262: 1
Mosquito: 1
N1K: 1
SBD: 1
TBM: 1
Tempest: 1
Typhoon: 1
As you can see, most types do not in fact have 3+ variants. While the list includes aircraft like the Ar234 that had no other variants, it also includes the Ju88 and Mosquito at one each, and those two aircraft had more variants that saw combat than any other types. The B-17, B-24, Ki-61 and N1K all have strong claims to other significant versions while only having one model of their types in the game.
I don't object to the P-51 getting another model, but don't paint it as a red heading step child as it certainly is not.