I think Monty Python already made a movie about that.
I've heard plenty of rumors about the next movie. I think that it's tentatively slated for sometime around 2011 or 2012. No William and Elizabeth, as their story has been wrapped. The main focus will be on Jack and Barbossa (supposedly). And yes, the Fountain of Youth is involved, but that will not be the main plot of the movie. However, I am not saying that this is definitive information, just what I have read. Scripts go through countless rewrites, so anything is possible.
Here's a link to one of the articles that I have read.
PotC 4
Rereading that cracks me up. Jules Verne. Atlantis.
More fish people!!