I almost always find myself in agreement with Cit's posts, but this one I would change slightly -
I'd love to see the P-39Q in US colors for a New Guinea campaign, but at the same time introduce the P-63 in VVS colors. The planes would share an awful lot of artwork.
One thing the US planeset doesn't have is a big-gun ground attack A/C. P-39 fills the bill nicely.