Reluctant Watcher

Xander's in some out-of-the-way place when he sees two guys fighting with swords. And then some other guy pulls him down and starts explaining about Immortals and recruiting him as a Watcher, and Xander can't turn him down because the only way to refuse is to say what he's really doing there. And with his preexisting knowledge of dead languages he gets tossed in Research despite his lack of college and runs into Adam Pierson. Possibly he's pre-Immortal.

All ideas are free for the taking.

Immortal Leverage

Some or all of the Leverage crew are Highlander-type Immortals. Possibly all of them except for Nate? Or all of them except for Nate, and then Nate dies his first death?

All ideas are free for the taking.

Saucy Halloween

On Halloween, Xander has to improvise a costume so he goes as an older version of himself which just happens to be Saucy!Xander. The lingering mental effects are greater because it's impossible to distinguish himself from himself in his mind. It's been more than fifty years of personally working to improve the whole snarled mess of demons and witches and slayers, building Saucy up to something great, and then suddenly all of his work is erased and he's back to square one with no real resources.

All ideas are free for the taking.

Pretender/Leverage crossover. Jarod and Leverage go after the same target and get in the way of each other's plans.

All ideas are free for the taking.