In another topic, I posted the following below. But no one said anything. So I'll make a poll instead. Because I really think these are cool ideas.
Maybe we would be encountering ghosts in XCOM 4. I know, that sounds like a crazy idea, but it doesn't need to be ghostbusters or anything like that. Just look at this video to see if you can think of something cool to do with ghosts:
Kara no Kyoukai:
When I see Kara no Kyoukai at 0:31-0:48, I think what if there was this Thief/Silent Hill-like city that was haunted by bad spirits. And like that in the Kara no Kyoukai video: You hunt them down. And.... that's all I have.
Other ghost ideas:
Maybe in XCOM 5 we would get Apocalypse 2, and show how human merged their Technology with that of the Dimension Aliens, and now everything in Mega-Primus looks like it has a face and teeth.
Or maybe we would be Researching these things in XCOM 6:
(I already showed this in my other topic 'Something Like Apocalypse....')
Maybe in XCOM 7 can be like Evangelion:
You know, I always thought Evangelion and Apocalypse had settings and plots near to each other. Both deals with defending a city from aliens.