My pitch made up while reading this thread so far:
Start: Divination has shown that something bad is on its way. Not worlds ending bad but enough to summon the heroes and let them get prepared. Perhaps even journey to where it is supposed to happen. Perhaps some interaction to gain resources, aid, other there things before the adventure. Make it so it can tie into the local groups ongoing NPCs if they have them.
Upon arrival: The sky opens up, something comes out and flys through the sky. Obviously artificial perhaps even clues to who made it in the description, but. it is large. It strikes the planet far enough away to allow some exploration.
Exploration: as the party approaches, they encounter things, monster or NPCs, that have come the the object. More time they spend looking, there clues they get, but perhaps clued into that the less time they have till something bad happens.
Basic plot: The thing that fell was a planes traveling ship. Think of an ocean liner for angels capable to traveling through the planes. It has been infested/attacked by creatures, perhaps infernal, perhaps that one horror race from 3.x BoVD (or was that the horror book?). Player must enter the ship, defend themselves, aid others, find out who is in charge, find out that those in charge (perhaps not angels, but good or neutral beings) and that if the situation can't be brought under control, they are going to blow up the ship to stop the threat. Players options are to aid the crew, stop the spawning of the enemy, help repair the ship, defeat the evil boss, thus the ship can leave. There is a clock, and if it runs out, then the self destruct starts and the players choice is to save what they can and get off. (think, if angels had a space ship and it was infested with xenomorphs while going through whatever was happening in the movie Event Horizon for feel).
ETA: for effect, no need to follow normal euclidean geometry for the ship. Perhaps all sides of the rooms are surfaces and one must thus navigate the ship to get to what is needed on the ceiling. Certainly larger on the inside than the outside.