I don't think they left a whole lot open for a new movie, they left some possibilities, but nothing as obvious as Pirates 2, for instance. I think they did it that way on purpose, to find out if they actually DO want to do another one. If they didn't, I'd say Pirates 3 is a perfect end to the series, but if they do, I'd say they could do it with little problems. Pirates 1 left no hint that there would be a 2nd movie, really, but they did it anyway.