Some good suggestions so far!
Personally, I'd like to see the following, payware, or freeware :-
(Below are already under development, and hopefully these will be released in the near future)!
Razbam Tempest V (development updates seem to have gone very quiet)

Vertigo B-25C/D (Still with programmer?)
Vertigo Mosquito FB.VI (Ditto?)
A2A MKV (Seems to have been put firmly on the backburner)
Warbirdsim P-39Q Airacobra (Actively under development it would seem)
After that, in no particular order: -
Mid to late P-40 models (Maybe the E, F, K, & M - hopefully FR will do the honours)
FSX Native BF109F & G series (FR hopefully)
Hispano HA-1112 Buchon (Another one for FR perhaps!)
Some later P-47D models (Bubbletop versions)
Bell P-63 (from Warbirdsim maybe?)
F4U-1/1A/FG1D, & maybe the later models as well (I still get a lot of enjoyment out of the A2A Aircraft Factory model)
Yak-3/9
Westland Whirlwind
Some later model Spitfires, would love a MK.XII, and then a IX/XVI & XIV with the cutback rear fuselage for a change (Maybe RealAir will oblige if they decide to update their existing models)
A decent Seafire, maybe a XV or XVII, but then I wouldn't say no to an later or earlier model either!
A new Hawker Hurricane package
Bound to be a few more, but that's more than enough for now!
Cheers
Paul