Following Andi Carrison’s excellent advice, maybe this is the time for Torchbearer? It can work pretty well as a short series since it’s centered around dungeons. Finish one dungeon, then gauge the group’s interest in another. Rinse and repeat.

You’ll want to get a few full cycles under your belt to get the full feel of the thing (i.e. a few Town phases), but it’s still got a short expiration date if you want it to. If you’re looking for published dungeons, the BWHQ guys have put out a few. But the game also works well with classic D&D adventures. Village of Hommlet and, especially, Keep on the Borderlands are well-suited to adaptation.