I've had a concept for a while to some form of "moé military," "schoolgirls with guns" type of thing involving girls doing tactical operations of some sort in a ruined city, and that eventually became what I call "School Girls' Battle Royale."
The concept is that, following a successfully-thwarted alien invasion, humanity has started training young people with exceptional abilities to fight off further invasions in case the aliens come back. Large schools were established, and one of these schools, as a major training exercise, has the school clubs form teams of four and face off in mock combat, battle royale style, in a nearby destroyed city.
So far, the concept involves >80 named characters, a city map, a spreadsheet detailing loadouts, and a 4-koma project that can be found here.