I got this idea from a very weird dream.
Basically, take the units, spells, and most of the mechanics from elemental. Now instead of a wasteland, transport it to an apartment building. Basically, I'm thinking an apartment building just floating in a void. Instead of exploring ruins or expanding cities, you explore apartments. Instead of buildings cities and revitalizing the wastes, you construct beacons of sorts that ward off the encroaching darkness from the void.
Whether or not this idea is possible depends entirely on the modding tools, would it support walls, multi-floor maps, or at the very least support modding those features in?
Have a whole bunch of other ideas refining the concept, (players start on the relatively untouched middle floors, main goal is mostly survival/returning home, not defeating opponents, the darkness encroaches from the top and bottom floors, fantasy thematic floors to make it even weirder, etc.) but that is the basic gist of it.