Some options:
- Call of Cthulhu, investigative horror - it’s very slick and commercial these days, but that may be an advantage in getting people on board. Lots of adventures available. Hardest thing to learn shifting from xD&D is that you sometimes have to avoid a fight.
- Lightweight indie games, which tend to consist of a one-shot adventure and a minimal system to support it. I have personal experience of Stone the Crows, Definitely Wizards, Overdue, and The Vampire Next Door, all of which were great fun – we didn’t take any of them very seriously.
What RPG system/game should I use for my one-shot? may also be of interest.