Turtledove’s World War could be a very good mod for G40, but it has the problem that one of the main alien landing groups starts at Poland (and holds the whole war as alien). That would totally mess any type of A&A mechanics, unless that you go ‘ahistorical’, give a token force for the Race at Poland and the focus at alien strongholds at southern hemisphere.
The alien invasion of US would be challenging for a modder, because you would have to mod USA’s territories. A mod best suit for AABattlemap or TripleA, but not for real board
Any case, it would need a VPs system to allow individual victories. And plenty of nukes. Also, different units and costs for Race and humans, with Race being uber-units but costly, and humans being crappy units but horribly cheap
For alien invasions in WWII, not only Turtledove, but also Hearts of Iron series and mods include some alien cheats or scenarios