I honestly think that the way Dead Man's Chest went, except for the cliffhanger and incomplete story, is what they should aim for, as it gives an impressive amount of creativity and vastly expands the mythology, gives Jack Sparrow a lot of screen time, while still not making him the main event, and we get a memorable villain in Davy Jones, with the Kraken alone being worth the ticket of entrance. The first film is a classic, but when they tried to mimick its simplicity in the 4th film it just ended up feeling boring.
I do think it's a shame each PotC since the 3rd has been facing problems during Development that lead into it taking such a long time, as well as Johny Depp seemingly losing interest. Because with every new instalment, they open as many new possiblities as they close the main plot, the 4th film basically ended with Jack Sparrow getting an Armada of ships, while Barbossa got Blackbeard's ship and sword. Along with past plots and other nautical tales, there are infinite possibilities, the last film almost felt like another episode of an ongoing story.
Some of the ideas i like the most are
Jolly Roger:
Adamastor, a large humanoid being that used to sink ships in the Cape of Good Hope:
Trident of Poseidon:
Another real life pirate like Captain Kidd being adapted, and turning the conclave on its head, as he leads into some kind of Pirate civil war. Could be interesting to see more united crews and a real battle between armadas.
Conquistadores as the next big group, the last film gave the spanish a role, now it wouldn't be a bad idea to go all out and have the Portugues and Spanish Empires play the roles the East India Trading Company had during the first Trilogy.
Another idea could be an adventure to find the treasure of Jewels tat controls the winds Angelica mentioned in the end of OST, but it was most likelly a lie. Another idea i like but wouldn't be my first choice is to take from Lovecraftian mythos.