I think, Spad, S.E.5 a and Albatros D would be great for the game.
Planes from Second World War were also very interesting, but I think, it is not good idea to mix them with First World War planes

Maybe you could think about two different battlefields from WWI and WW2 with possibility to choose one of them, if player community grows

But it is only my idea. I can’t imagine how expensive the development of these features would be.

I understand that you are trying to improve the game in way, which will attract more players, but I’m happy, that you are not thinking about adding F-16 with guided missiles.
I like machine-gun fight much more then fire and forgot combat