My thing is a WoTC 5e-compatible thing; "Old Smoke" is a contemporary London-based project I'm working on, where players navigate an arcane, alternate-history version of the English capital:
Centuries-old criminal institutions, secret societies maintaining magical balance, randomised magical effects, and a big, wobbly hole in reality that London itself is scared of called "The Noose". Hooray!
There's also a random encounter generator, and a lore based on my own born-Londoner knowledge, meaning looking up any of the events or names in Old Smoke's history will turn up results that could be a cover-up of what Old Smoke suggests took place. So there's that. :D