(I'm not sure if 3-way battles are possible in OpenXCOM.)
There should be an EXALT/Cult Of Sirius mode where it is focused on mainly combating humans(XCOM). There should be some focus on urban terrain to fight in, such as an apartment block or something. To keep it simple, the only enemies are humans, as in XCOM and whatever local forces there are(Police/Military/SWAT/Armed militia/Occasional Civvie With a Gun/etc).
The way to win is to assault the main XCOM base, where you will kill the XCOM commander(Not just any XCOM commander. Think Enemy Unknown). You will be able to do terror missions, infiltration, base attacks on XCOM, and so on. Basically, the "alien missions" that the aliens do? You should be able to do them, but a human version of it(Terror/Base attack/and so on.) Ground missions such as Terror missions should come in two stages. The first stage of a terror mission gives you a limited number of turns to do the job before XCOM arrives. To win, slaughter XCOM and everyone else.
Special missions such as Terror missions or infiltration missions should require SOME resource or something to prevent the player from endlessly spamming them.
Implement this however you like, because I can easily come up with ideas like this, but when I delve into how the game mechanics should work............
