Captain Barabossa was one of the great things about POTC, he was the only pirate after Captain Jack, that actually looked like a pirate. If they were to make a sequel he has to be in it.
The current plot details state that the movie is about the chase for the fountain of youth between both Captain's and it also may have something to do with the a story of the same name.
When interviewed about the subject Rush had a lot of information to give us on the status of the projects: