1: Volcano: It's set in LA, it's got a volcano, what more do you want? Maybe Wolfram and Hart summon it in some way, maybe it's what's left of the area around the Hyperion after Not Fade Away, maybe mad cultists decide that the One Ring (currently incarnated as Dawn Summers) needs to be sacrified there.
Incidentally, there's a lovely panning shot right at the end of this otherwise lacklustre film that shows LA with the volcano, I'd really like to see it in an Angel fan-vid some time...
2: Good Omens: I think it's canon that Robin Wood was bought up by his mother's Watcher - whose name was Crowley. Apologies if this isn't so, but if it is there are some interesting plot possibilities, if only one or another Crowley picking up the White Courtesy Phone at an airport and hearing something intended for the other one.
3: Broken Arrow: The Initiative is the nearest military unit when the nukes are nicked. And demonic forces are also after them.
4: Prehistoric Park: The First Slayer stumbles across the time portal while it's open, and steps forward to the present day. Chaos and time paradoxes ensue.
5: The Sopranos / Highlander: The family has trouble with a vampire gang moving in on their territory, but someone half-remembers a story of an old debt owed to them by a group called the Watchers. Unfortunately the Watchers they eventually locate are the wrong ones...
Really doubt that I want to write any of these, but someone might.