I also said that Jack would be in the film as well. Here's how I picture a fourth film could work...
Jack and Barbossa have been chasing each other across the seas, fighting over the Black Pearl, the map, etc. So far, neither one of them has managed to get to the Fountain of Youth. It's about three years after the end of AWE, so Will and Lizzie's child is about two and a half...
In the middle of Jack and Barbossa snarking at each other, a pirate ship rolls up to them with dire news; Elizabeth has been attacked while on a return trip to Shipwreck Island. She was injured badly, and William the Third is now 'a guest' aboard the enemy ships.
Thinking that they've killed her, the pirates have gone to Shipwreck Island to declare that a new King be elected.
Jack and Barbossa stop screwing around and get thier butts back to Shipwreck Island, where they get worse news; Will's heart has been stolen by some other-worldly creatures, and Elizabeth's injuries are life-threatening.
Meanwhile, in the Land of the Dead, Will is having his own problems, having irritated a goddess or other supernatural being because he refuses romantic overtures or refuses to be swayed into straying from his proper duty.
In order to seek to control of Will and the Dutchman, this creature has taken Will's heart, and they begin to change Will from being honorable and noble into something twisted and evil, and worse than Davy Jones.
Calypso pops up to ask Jack and Barbossa for help, finds out about Elizabeth's injuries (which can only conviently be healed in the Land of the Dead by some sorcerous), and helps them get to the Land of the Dead to 1. Save Will, 2. Save Liz 3. Restore order to the Land of the Dead.
Meanwhile, Daddy Teague holds political type meetings etc to hold off the kidnappers until Elizabeth has been healed and can resume her rightful place.
More exciting stuff happens which I'm too lazy to think of right now, everyone is saved, Will becomes good again, they come back, kick the bad pirates' asses, and Liz and Will manage to have a whole evening together, where Will meets his son and he gets to snog Lizzie. Then Will says good-bye to everyone, and heads back to fulfill his duty as a non-evil person ferrying souls.
Barbossa and Jack decide they work very well as a team and decide to go find the Fountain of Youth together, so they can have some and also take some back to Lizzie. As they sail out of Shipwreck Harbor, they begin to argue over who is rightful captain of the ship, and the credits roll.
So yeah, you could totally work out all the characters being in this film again without having it be 10 years in the future, and I only spent a few minutes on this.
