Ever since seeing Danmachi, I've wanted a procedurally generated, near infinite, large-floor dungeon where each floor or group of floors has/have its own look, inhabitants, and floor boss. Something like floors 1-5 using Black Shroud assets while floor 6-10 use Thanalan assets. At this point we've got enough world (hunt), dungeon, raid, and trial bosses that they can easily populate many, many floors.

Scaling would be a must; scaling to the party's level and number would be very important. It wouldn't be be cool to put a raid-level boss into an instance with four players and vice versa for a 24 player group. It would be pretty awesome to have the majority of floors be 4 player, with 8 player group bosses (floors 1-10 etc), and 24 player bosses every 20 levels or so.

PoTD and HoH are good proofs of concept/starts, but the floors are generally small and basic in their layout. Having floors that are 1/4-1/2 the size of a zone with a real sense of sandbox exploration with challenging monsters would be awesome!

I'm not sure if this is practical or doable, but since this is a pipe dream thread, you've got my pipe dream.