Not so long ago I made a mod for the Open General platform (Turn based engine of the Panzer General 2 but wastly extended and expanded and as fully moddable as Open X-com) what is more or less based upon X-com. It interprets my version of the aliens history as how they crossed through the space and subjugated many races to twist them to those abominations what appeared on Earth finally.
The mod's installer's link and some description could be found here:
https://panzercentral.com/forum/viewtopic.php?f=132&t=50241But before you do anything with the downloaded exe (what is the installer) go here for the basic Open General and a very good description how to use, set up or mod it:
https://luis-guzman.com/Hope you will like the mod and it's story!
P.S.: Here are some screenshots for you to help you decide toward a big "Yes, I wanna play it now!"