I have some ideas, sadly I know almost no C++ beyond 'Hello World' as I've just started learning. Thus, i cannot make them happen. But I'd like to run them by coders of CE.
(I still have not started a game in Xenonauts yet because I am feeling very ill. I haven't gotten work done that needs doing either.)
Anyway, in XCOM Enemy within, you see news reels which say things like "militias are reported to be attacking aliens worldwide." Also, since you can sell alien items on the grey market, and sometimes governments will actually request corpses or any kind of alien salvage, I think some reasonable assumptions are in order. (Yes I know, Xenonauts is not XCOM, but we all know it is designed to be a successor.)
Each government is obviously doing STEM research on their own, as well as military planning, independent of XCOM/Xenonauts. Therefore, I think it would be very interesting to be able to acquire some direct assistance from any nation which has considerable military/economic power. By assistance I mean providing garrison troops, military equipment and personnel during battles. I think it would be DOUBLY interesting if friendly AI could command their own troops, though the more obvious solution is that you are given command. As for militias, they'd obviously exist, and I think it would be interesting if you could recruit civilians with combat/ police training / experience with firearms, as free (but lower quality) garrison troops, as they would be volunteers.
It might also be interesting to be able to request military personnel from one of the independent governments, as in, request a certain amount of troops or equipment to be temporarily under your command. If there is a UFO, you can request local fighters to scramble to provide additional aerial firepower.