I think all the planes that would be popular in the Main Arena are already in the game. Thus, whatever is added likely isn't going to change much what people see flying around in the Main Arena.
However, there are planes that can make a significant difference to special events: the He 111 (a major aircraft of Battle of Britain, Ju 88 isn't a good substitute), Pe-2 (a major Soviet attack plane, 11,000 produced, no good substitute), G4M (the main Japanese bomber, no good substitute), LaGG-3 (we need early-war Soviet fighters for eastern-front events), etc.
For special events, the P-39 and B-25 were excellent additions (and I do see them around even in the Late War Arena).