Thanks for the post Sour Kraut.
This seems most likely to me.
As for New Orleans - it's seems so simple once it's been stated...because where, in the world of the Disney Parks, is Pirates of the Caribbean housed? Why it's in Port New Orleans! Another lovely callback to the origins of this franchise.
As for the Fountain of Youth? Well, we had that obvious clue at the end of AWE. And, if you have the POTC: DMC 2 disc set, check out the extra feature on disc 2 "Charting the Return".
In an interview shown to be done in October of 2004 Gore Verbinski recalls a meeting he had with Jerry Bruckheimer a year earlier and Gore mentioned that there were many ideas for plot lines discussed for sequels. The only plot line he actually names is the Fountain of Youth.
As for Will and Elizabeth - Sour Kraut's Wikipedia article states that Keira has no intention of doing another POTC movie. She has stated that publically, and has also stated that she's considering giving up acting altogether.
One of the writers, and I'm sorry I don't remember if it was Ted or Terry, stated on their online Q&A type blog that their backstory of the Dutchman curse went along these lines: Will & Elizabeth had that one afternoon on shore before Will had to take over duties on the Dutchman for 10 years. At the end of that time he returned to shore, where Elizabeth, and their son, waited faithfully. Thus...in the writer's story universe...that ended Will's required duty on the Dutchman and left him and Elizabeth and their son free to enjoy the rest of their lives together. The writer hinted that that effectively ended any useful storyline for Will & Elizabeth in the POTC universe.
Remember Tia Dalma had not held up her end of the bargain with Davy Jones, and been unfaithful, thus began the Dutchman curse. (And we do realize of course that she was unfaithful to Davy Jones with none other than our witty Jack, thereby begetting the bad blood between those two).
Although it wasn't discussed, the only logical progression I can see from that is that it's now Calypso/Tia Dalma's duty to find a replacement for Will. Presumably sosmeone else had that job before Davy Jones buggered it up, and since Calypso chose him for the job, it stands to reason she would choose the next candidate. Remember, it was only Davy Jones' perversion of his mission that led to the whole "stab the beating heart/the Dutchman requires a beating heart" dilemma. Once Will fulfilled his 10 year tour of duty, and Izzie was faithful, they're free to go about their business and let someone else take over for 10 years.
At least that was my understanding of how the writer's intended it. In fact whichever writer it was said they were unhappy that the explanation did not make it to film and left so many unanswered questions in the minds of the filmgoers.
I've heard nothing about whether or not Geoffery Rush has agreed to POTC 4, but I'm thinking he's critical for the movie. I've heard rumors that there's a young actor being sought for the movie. Some reports have it that the younger man will play Jack's younger brother. Others believe it could be for flashback purposes, while others still speculate it could be related to the Fountain of Youth, i.e.; Jack drinks a bit too much water.
Who knows? 3 whole years to develop theories.
