I picked up Ebonclad, which is a series of short adventures and a sourcebook for a thieves' guild. And I found a screaming deal (on eBay) for remaindered copies of the 5e Abomination Vaults adventure path ($21 and change, free shipping). I think I can mash up both of those and come out with something really neat.
[Edit: I was on my phone when I typed that earlier. Additional thoughts follow.]
I want to run the Ebonclad adventures as a lead in to the dungeon, which I'll place beneath the city streets of Kintalla (the city that comes packaged with Ebonclad). I'll have the PCs start as new-ish initiates into the thieves' guild and give them the opportunity to feel out the city, make some contacts, etc.
I plan to give the guild a sacred duty to ward the titular Abomination Vaults, sending off the PCs to investigate when the weirdness first starts off (assuming that's doable within the framework of the adventure path; I haven't got it in my hands yet, of course). The PCs (and the guild) will find themselves thrust into the unlikely role of heroes (I may have watched Thunderbolts recently).
Just a jumping off point, of course. All subject to change.