It's hard to think of a plot that would make sense... why would everyone get back together again?
Maybe Monica's been kidnapped or something, and so the gang gets back together to track her down. That doesn't really seem like a good premise for a movie based on a sitcom.
I read one of the Friends actors say that if they had a good screenplay, they'd do a movie... so if we come up with a good idea, maybe we could help bring back Friends! Haha not really, but man it would be fun. Or maybe it would be awful.
If they made a Friends movie, what might the plot be?