i also feel the apocalypse is memorable, if they can just skinned it to HD and eye-candy, it would be a best seller.
Apocalypse really did a good job with the X-Com universe.
The alien biology and lifecycle and roles they play all links up.
Multiworm->Hyperworm->Chrysalis->Any Aliens, etc...
The Cityscape UFO battles were so epic!
No lame screen just to see UFO shooting the planes but a full city map to chase and shoot down the UFOs. Give your groups of aircraft formation and entrap the UFO escape path as they make their way to the portals.
Then there's the organization relationship thing.
Marsec hates Megapol, General Metro hates Superdynamics, etc....
First thing I do is whack Cult of Sirius!
I love if they went back for a remake of this!
10/10 would buy!