My next fantasy RPG campaign...maybe

So I've been tossing around ideas for the fantasy campaign I'd like to run after the current one is done.  This one still has a goodly number of sessions ahead but I like to play with game ideas and the sooner I decide the more prepared I will be when it's time.  I was thinking maybe a megadungeon.  I've never run one or played in one but the idea has always attracted me: What if you could explore all of the Mines of Moria?  Or maybe a big city campaign where all the action takes place in a sprawling metropolis with occasional side jaunts outside. Or something on the Astral Plane--that's a very cool setting--possibly combined with a "Fantasy Battlestar Galactia" concept, like the creative Last Fleet campaign Lowell Francis posted about on his Age of Ravens blog.

So many ideas...so little time.  Or...maybe I could do ALL OF THE ABOVE AT ONCE!

Okay, here's how it would go.  A long time ago a group of peoples were faced with the destruction of their home plane/planet by some vicious enemy.  They were able to build (or already had) special ships/mounts capable of carrying them to the Astral Plane and thus escaped.  But the enemy sent pursuers.  The survivor fleet fought and fled deep into the astral void, finally ending up at a immense floating tomb-like structure associated with a long-forgotten dead (or maybe not) deity.  For some reason the pursuers were afraid of the tomb/building and retreated, but today occasionally still send scouts.  The refugees found that the massive structure had bits they could mine/gather/harvest(?) to survive.  Eventually they found out the hard way that the tomb-thing contained lethal dangers.  The leaders finally forbade all from delving into the place so that no threats might be aroused or released.  Only the outer portions could be used for the activities upon which the community depended  Over time the old fleet ships were expanded, linked, and then increased in number by building additional ships/hulks. Later traders/explorers from distant parts of the Astral Plane discovered the community and some low-volume trade opened up, allowing them to prosper by selling the various rare products obtained from the outside of the tomb.

Today the fleet is a sprawling connected tangle of ships and other floating structures, some permanently connected and some not.  There is a lot going on in the "town" and outsiders occasionally visit.  Inside the tomb-thing is a megadungeon awaiting foolhardy adventurers, possibly multidimensional (like a Tardis) but definitely with a Deep Dark Secret buried deep within it.