Speculation has already begun that the subtitle "On Stranger Tides" refers to author Tim Powers' novel with the same name. The following is the official description of the book:
Puppeteer John Chandagnac, who was sailing to Jamaica to get revenge on the uncle who had stolen his father's inheritance, has no choice but to join the buccaneers who have taken him prisoner--and soon, known now as the pirate Jack Shandy, he finds himself learning to survive in a treacherous new world of cutlass-fights, sea-battles, and voodoo magic on sun-blinded tropical islands. The legendary Blackbeard is assembling a ruthless navy of pirates living and undead to voyage to the fabled Fountain of Youth, and Jack Shandy must use magic, swordsmanship, and even his puppeteer skills to free himself and the girl he has fallen in love with from Blackbeard's deadly supernatural domination.
At the end of the third "Pirates" movie, it is shown that both Barbossa (Geoffrey Rush) and Jack Sparrow (Johnny Depp) are next eyeing the Fountain of Youth. Barbossa has "stolen" the Black Pearl back from Sparrow, but Sparrow has cut out the most important part of Barbossa's map to the Fountain of Youth.
If they will be adapting the book, will Sparrow and Barbossa have to team up again against Blackbeard? We'll have to wait and see how it will turn out.