Here's my submission:
A Cloud Giant's floating castle that has been overrun by 5D12 goblins. Their leader is a 4 HD goblin named Skreek Von Fizzle that saves as a Magic-User and knows random spells. The goblins have no idea how to control the castle so it has been floating aimlessly for weeks. The goblins have subsisted on migratory birds but they are starting to get bored and 25% are plotting a mutiny. 1-4 chance that the first goblin(s) the party encounters are rebellious and may ask for help overthrowing Skreek. Loyalists will try to bring the party to their leader who may want to negotiate with the party to learn how to pilot the castle. Or imprison them as ransom. Or just eat them.
The original owner of the castle died and its remains make up Skreek's throne.
Generate a random set of maps with donjon.bin.sh to represent the castle, which is 2d3 floors tall.
I want to try and do something like this regularly, so look forward to more!