i'd be lying if i said i hadn't thought about it.
i'd do it as a series of missions set up as a string of "nextStage"s, via new battle mode, and skip the geoscape entirely.
i'd put armories and weapons caches in various maps, so the player can resupply mid-mission.
i'd tie everything together with cutscenes.
i wouldn't worry about a lack of variety of enemies in the slightest.
the only part i'd really have trouble with would be the climactic battle with the queen and the power loader.
another idea i've been toying with, which is in a fairly similar vein, would be a space hulk mod, with genestealers running around in tight corridors.