While I really do want to see the earlier stuff like the Airco and the Nieuport, heck, all the way back to the Eindecker... I have to agree with Wmaker's idea. Flesh out the late war, then bring in the earlier stuff. Albatros D.Va, Spad XIII, since we've got the D VII, maybe the Sopwith Snipe for parity?
Not sure what my fourth would be.
Wiley.