New Horizons is definitely worth checking out. It is based on POTC but it's just as standalone like the rest of the games (you need the Maelstrom engine but that's it). To install it you need to buy the Maelstrom engine and link the Maelstrom engine to the game. There is a great tutorial on Youtube on how to do it, just search for "Maelstrom Engine installation guide".
Quite a few islands have been added compared to the original POTC and the map is much larger but in terms of nations there are still the four standard ones + Pirates and Sweden (if you activate it in the settings). But only the four standard ones and Pirates have settlements.
Hope this info helps.
