Next project! Everyone has their megadungeon, right? Well, this is going to be my take (sort of) for the Basic/Expert system.
You're looking at an adventure that's going to cover at least 5 character levels, but can be split easily into two distinct parts so if you want to skip the Basic (1-3) section, that's fine!
It's going to be a hex-crawly adventure through a cave network (think Journey to the Centre of the Earth) for part 1 - a real spelunking feel with lots of interesting mini dungeons and encounters.
Then part 2 is the main event dungeon - an entire lost ruined city! Verticality, monster factions, hoards of treasure and something deeply dangerous and ever present, lurking in the dark....
I hope to share more as things begin to take shape!