I've just been to see AWE so my minds brimming with ideas, I have been wondering for a while if the mainquest could be extended, rather than replaced and preserves the identity of the player.
My idea is this
The player gets back from Keal-Roa and learns that sliehard has been replaced by a new governor arriving from England (governor swann) with help from captain Norrington. You as the player meet Jack sparrow in a pub on Tortuga and the pearl gets mentioned. You tell sparrow that you had sunk it and he attacks you for "blowing holes in his ship" After beating him (but not killing) he tells you that it wont stop the pearl for long. If Barbossa and his crew truly were cursed they could just raise and repair the pearl (backed up by the ending FMV from the old mainquest)
Sparrow leaves and you see no more of him for now.
Later on (after finishing quite a few sidequests) you arrive in Redmond, and after spending an evening in the pub the settlement is attacked by the pearl. You run to the fort through waves of pirates (your sword knocking them out due to residual energy from the native artefact). There you meet Commodore Norrington who completely disregards your warnings about the pearl and tells you to stay in the fort while his men clean up the mess.
In the morning you are confronted by a frantic will who explains that they have taken Elizabeth. after the Norrington no brash action speech he rushes back off.
When you go to port you realise that something is up on your ship and proceed to the deck. Jack and Will are abord. you agree to help them. you agree to meet up with them in Tortuga and after disabling the rudder chain you abandon ship to alert Norrigton. After Norrington goes out in the Interceptor to recapture your ship will and jack escape with the Interceptor. Norrington gives you your ship back and then sets out in the Dauntless to attempt to find Elizabeth who he professes to love.
When you rondavou with the Interceptor off Tortuga it joins your fleet fully crewed and ready to go. Jack gives you the bearings to the Isla de Muerta. Using the item teleports you to the new sea area where you enter the treasure cave from the film. You help will rescue Elizabeth (Jack "falls" behind) The interseptor takes off back to redmond and you go your separate ways.
Back at Redmond you find not the Interceptor but the Dauntless. Elizabeth tells you they have Will and Norrington wants your help as rumours are abroad that you "Sunk" the pearl once before. The dauntless joins your fleet and you sail to the Isla de Muerta using the bearings item from earlier
Jack goes in to convince Barbossa to come out. after a short battle on the deck of your ship with cursed pirates you go into the cave yourself to battle Barbossa in the flesh. The battle should consist of a challenge of a stay alive nature. you must fight Barbossa for long enough to allow will and jack to return the coins they stole. when this is done Barbossa dies and you go back outside. the pearl has gone. only your fleet and the Dauntless remains. Jack goes back to be hung. He gets free with help from you, will and Elizabeth. jack and you flee Redmond and then sparrow offers to join your fleet with the pearl. If you accept he joins (but you shouldn't be able to separate him from his ship.) If you decline he goes to the Isla de Muerta and waits for there (so you can recruit him at a later date)
A while later (if you recruit him) Jack tells you he needs to leave (and does so) When you next sail to Redmond you find Beckett in charge, Will and Norrington gone and Elizabeth in prison. Governor Swann takes you to one side and talks about taking Elizabeth to England for him. You agree. When waiting down on the dock Merser comes up and stabs you, leaving you for dead. You wake up in the tavern and learn from the innkeeper that the Governor is in prison and Elizabeth has escaped off Island after stealing a letter of Marque. When you get back to your ship you find a letter from Elizabeth in you cabin. It tells you that if you are still alive to meet her in Tortuga.
When leaving Redmond the Flying Dutchman surfaces and attacks you (due to your passed dealings with sparrow) When it gets low on heath it dives again. When you reach Tortuga you meet Jack and Elizabeth. The pearl joins your fleet once more. You then visit a string of islands. On each one you fight a group of pirates but each time find your query is not the dead man's chest but in fact a stash of rum (some of these fights should include sea battles). Eventually after much persuasion jack hands the compass to Elizabeth and you find the DM'sC straight away (along with Will). Finding the chest leads to a land battle against pintell and raggettii. (which they survive) followed by a land battle against Davy Jones crew.
Back at sea you need to flee the Dutchman (as its immortal) but the pearl is sunk (in an FMV) by the Kraken (or is it Kraaken of Kroken) the crew of the pearl (Will, Elizabeth, Gibbs, Pintel, Ragetti, Cotton (and his parrot), and Marty) escape aboard your ship. Together you go to meet Tia Dalma who proposes you travel to worlds end. She also tells you that any ships you take wont be coming back, and so you may wish to find a less valuable ship than your own. Barbossa is revealed and he and Tia Dalma join your crew.
(Somehow) you now need to get to Singapore where you attempt to recruit one of the pirate lords (cant remember his name) and it ends with a massive land battle with Beckett's troops.
You then have to sail of the edge of the world with the Junk (using the chart to get to the edge of the world itself, an FMV showing you go over) You rescue jack, and then FMV back.
After the betrayal at the hands of the pirate lord (and will) you face a boarding to protect the pearl against Beckett.
You travel to shipwreck cove with Jack where you find the pirate lords (one of them should be the chap from QC so its not completely unrelated to the POTC we have been playing) have gathered, with Elizabeth replacing the lord from Singapore. Elizabeth gets voted king (wouldn't that be queen) of the pirates and war is declared. When you leave you should enter a massive ship battle where all ships are neutral and standing still but the Dutchman, It should be stormy, and you should battle for a while until the game initiates boarding. you should repel boarders to the pearl and then board the Dutchman yourself. You should fight off the fish pirates and Davy Jones. When Davy Jones gets to critical heath he should kill Will but not before the heart gets stabbed.
You escape to the pearl with Jack and Elizabeth. (the Dutchman should sink).
Then the fun part, a massive ship to ship battle, pirates vs navy with help from a resurfaced dutchman.
For finishing this monumentus quest you should get, Barbossa and Jack as officers (you should need to keep an eye on them) and the Pearl as a ship. In addition you should get an artifact that can be used once every year which summons the flying Dutchman to aid you. You should also gain perminant access to shipwreck cove, an always friendly settlement, even when you are at war with pirates. It should function as a extra powerful fort with one of the best shipyards, a store where the rum flows freely and the most pirately tavern out there.
Thats my idea anyway. Some people (the ones who have done the mainquest so many times already, and the ones who don't want to play it through again whenever they start a new game) might want a build trigger with options to skip the mainquest and start straight with the extension.
Also the pearl would need to be unboardable during the original mainquest encounter.
I can see some people not liking this but I needed to get it out of my head.