My 'freebie' suggestions:
C-47A
F4F-4
P-40B
SBD-5
Hurricane Mk I
Spitfire Mk I
Bf 109E-4
Fi 156
He 111H
Ju 87D-3
B5N2
D3A1
A6M2
I-16
Jeep
M-3
SdKfz 251
Elco 80' PT Boat
If I might offer my humble opinion I think that list, taken just for example, is far too comprehensive and any new revenue generated would likely be offset by current players taking a temporary payment hiatus while you would still see them playing in the arena.
I was thinking of something closer to they way Rise of Flight offers one central and one allied fighter for free in multiplayer. Everything else is paid. When I started playing that game I flew those two planes for nearly six months before making my first purchase. Now, a few years later, I've paid for just about every plane, map, or upgrade available in the game store.
I'm of the opinion that two or three early war planes available in the MA is easily enough content to provide a free player plenty of time to decide if they'd like to support the game, as well as boost player numbers/action/targets for the paying members. Remember, everything is free to try offline and in the training arena. Free rides in the Main Arena should exist only to provide a window into MA multiplayer action (the games only real lure) for more than two weeks.
Building personal relationships with other players is the BIGGEST hook any multiplayer game has, but these things take time.