Inspired by my not knowing what was going on in The Wheel Of Time rights-securing pilot, I got the idea that one of an old hero’s enemies came to visit, not to gloat about his belated victory, but to seek his help.
A villain has spotted that a much bigger threat is on the way, so putting personal issues aside, contacts the forces of good. Can they fight together? Can the heroes work with an archenemy while always expecting their Sudden But Inevitable Betrayal? How well do their concerns really align, and how long until Spike grabs Dru and abandons the heroes to the final battle?
(This doesn’t literally work with Mordor, obviously, but I saw a pun...)
And does it depend if the villain is a player character?