Issuing light perks(4~5 points?) on the planes P-51D, Bf109G-10, Fw190D-9, La-7, and the N1K2-J will each greatly increase the usage of P-51B, Fw190A-5 and A-8, La-5FN, Bf109G-2 and G-6, and the Ki-61 IMO. Also, the perks for the Spit14 and F4U-4 should be downed a bit.. (20~30s?).
Ofcourse, filling out some of the gaps in the planeset(the gap between the Bf109G-6 and the Bf109G-10, and the SpitfireMkIX and the Spitfire MkXIV seems to be the largest) will also help.
So.. give the RAF a 1944 Spit9, give the LW a 1944 Bf109G-6(or a G-14

), maybe a Ki-44 to the IJAAF... and the arena would be full of mid war planes. Nicely matched, balanced, free planes existing for all countries.. and some of the monstrous performers a bit rare in comparison, the even better performers like the Spit14 and F4U-4 would be more rare, and the Tempy/262 planes would be just as now very rare to find.
This would help the arena to boast some nice diversity.
Either that, or everyone deprived of their "dweeb machines" end up in Spit9s and N1K2s, and balance is even worse off!
