I have some thoughts about this:
I think that there should be a fourth film. The third ended in such a way that it merited another to clean up the mess they started. If they do a fourth, it should be the rest of the war they started. The EIC lost the first battle and Beckett, but there is no doubt that the Company officials in London would send another agent... perhaps someone with more military bearing than Beckett, which would replace both Beckett's lead enemy character and Norrington's military character.
But to bring light to what now looks like another drama, a new player has entered the field. This new addition to the film would give Jack's side more hope and better odds to make it more of a swashbuckling battle than the ominous lopsided fight seen in At World's End.
<b>Barbossa:</b> "This battle may be won, but this war is far from over."
Scenes of the Brethren Court's fleet regrouping and beginning much needed repairs.
<b>Barbossa:</b> "And this time, the enemy will return with unparalleled resolve."
Scenes of a massive fleet in line astern. Scene of a heavily decorated EIC admiral on the quarterdeck of a ship.
<b>Barbossa:</b> "There is not much hope left for us..."
Scene of <i>Black Pearl</i> sailing off. Scene of Jack alone in his dingy.
<b>Jack:</b> "Perhaps there is a chance..."
Scene of Jack's dingy floating into the shadow of a ship, camera pulls out to reveal a frigate.
<b>Jack:</b> "The death of piracy may very well be just over the horizon..."
(Continuation of previous shot.) Camera pulls out further to reveal the ships flag: Red, white, and blue stripes and a blue inset with white stars.
<b>Jack:</b> "But the fate of the free man isn't."
Americans. It may not seem all too appealing to Europeans, but the science-fiction factor isin't there and the numbers of the opposing sides don't make it seem desperate and dramatic. The Continental Navy was certainly viewed as a band of pirates and this could give Jack his chance to do a "spec of honest pirating" as a privateer with the colonials.
Only problem with this idea is how the rest of the world sees the American Revolution, if they even know about it, and the fact that is gets a little too close to mixing PotC with Master and Commander, which is more of a navy fanatic, historical buff fiml than a swashbuckling pirate adventure.
The thought of a prequel is appealing, but they would have to make one from scratch and ignore the books written presently. They could go deep into how Jack marked Beckett, the transformation of the <i>Wicked Wench</i> to the <i>Black Pearl</i>, and the deal behind Davy Jones and Jack's soul... which is a very big part of the present movies since its what DMC revolves around and leads us to AWE. We could also get more character development from Captain Teague, Bootstrap, and Tia Dalma. We could also see the brush with the EIC, sacking Nassau, Jack's previous visit to Singapore and Sao Feng's reasons for hating him, and why Norrington was so passionate about catching the <i>Pearl</i>.
What say you?
(PS: There was a news article that comfirmed Disney's starting of PotC4 in 2009.)