Short, mission based 'Whoever is available' gaming where Aliens are invading earth and its up to a small non-copyright infringing anti-alien military group to fend them off. Fairly high PC turnover because the aliens either have more high tech gear or are genetically superior to humanity.
Mission Failure/Partial/Sucsess grants bonuses on post game research rolls, which add 'Prestige Classes' like Cybernetics (BELLATOR IN MACHINA MOTHERFUCKER) or DNA Manipulation, better equipment and advanced tactics to the basic character classes.
The nature of the aliens would be a 'Discover during play' style dealie with the players basically choosing their own enemies and the ST working out research projects for each alien type.
Since each mission would be a single session, it doesn't matter who turns up to the game, but previous players who's characters havn't died get to use their previous character sheets alongside green rookies. Just like X-Com
Dungeon World crossed with X-Com, basically.