A campaign nobody would want to play in (at least after the first session):
The PCs, via a hard railroad, meet in a tavern. They remain in that tavern, because no matter what they do they can only go to a) the kitchen, b) the washroom, and c) three guest rooms upstairs. That is the extent of their available world, and they cannot leave it by any means. The only NPCs they can interact with are a) the barkeep (a dull and bitter man), b) the cook and assistant in the kitchen (the cook is fussy to a fault, the assistant is dumb as a post), c) two other customers in the tavern who are simple common farmers with nothing of interest to say, and c) the upstairs valet, who is mute. The entirety of the treasure in this building consists of 15 s.p. 8 c.p. in the bar till and about the same again divided between the occupants and some forgotten corners of the rooms upstairs, plus whatever the PCs have of their own. Looking out the windows shows an endless prairie of wheat fields in all directions except a rough cart track leaving to the north, all suitably narrated to reflect the time of year.
How to (maybe?) make it awesome:
The tavern building is mobile - it can 'walk', it can fly, it can go underwater or travel through interstellar space, etc., and it can communicate externally as directed, but it can't in any way think for itself - but the PCs have to figure this all out. After that the goal of the PCs is not to themselves adventure but to a) learn how to guide/steer/operate the building, b) to teach the building how to do certain things e.g. fight for itself, conceal itself, etc., and then c) to use their collective skills and abilities to guide it through a series of adventures and-or to a series of interesting places. Their means of interacting with said adventures and places is the building: the PCs tell it what to do and-or 'say' and it does so.
Lanefan