I agree that the Superfortress would be a nice addition to the planeset, but there are other planes that I would consider much higher on the list of "must-haves".
Brewster Buffalo
Mustang I-A (early P-51 with 4 Hispanos, probably a perk ride like the C-Hog)
Spitfire Mark IX LF
for Japan, the Oscar and Frank (don't remember model numbers)
P-40K or N (just 'cuz I'm a Warhawk nut and the P-40B and E are either flat inferior or undermodelled)
P-47N
P-39 Airacobra
Mig-3
the Hurri with 12 .303s (great for scenarios)
Hawker Fury/Sea Fury
The, maybe the B-29, but PLEASE, no nukes. That would totally unbalance the arenas.