I'd like the hypothetical expansion (if it's ever coming) to revisit a core component of EU/EW: UFOs. I spent a lot of time anxiously crawling through UFOs in the first XCOM, and miss that feeling. I understand the reasoning for why UFOs aren't as common in XCOM 2, since the aliens have ground bases and what seems to be a ubiquitous railroad system. But since there are still flying saucers in the world of XCOM 2, why not explore this more in the expansion? I'd especially love to fight through giant flying fortresses again.

Yup. I would love for Firaxis to take on the idea of the religious imagery used in X2 and make that the theme of an expansion in combination with the UFOs. Initially I thought it might be too "controversial" I guess? But after seeing that Mass Effect Andromeda is pulling in that sort of theme and it's almost about as mass-market appeal as you can get at this time, I figure XCOM could do it.

Basically the ideas that the UFOs allow the "chosen" to "ascend". Literally uplifting those faithful to ADVENT's religion into their Temple Ships. We've seen that there are ADVENT churches in XCOM2, I'd love to see the idea of ADVENT cultivating the most loyal and turning them into cybernetic/mutant fanatics. Touch on some of the cult/Scientology conspiracy type stuff.

There's already a lot of elements there to be honest. You could easily mix in Archons with whatever new alien or mutant/Nephailim are added, maybe even some feeble old-school Ethereal. You could also have units like ADVENT Inquisitors who hunt down heretics to their religion (and are secretly looking for psionic-sensitive candidates for harvesting).