The Pirates franchise was indeed very popular in Japan. Box office wise, it was on par with Jurassic Park or Star Wars at its peak and lagging noticeably behind only the Harry Potter franchise. You make a great point about the Pirates summer show. I've never seen such a huge and energized crowd at one of the seasonal event shows. I think they originally only intended to use the summer event to promote the fifth Pirates film, but its popularity caused it to return this year (and probably many others). No question, it's the most successful summer event they've ever done.
Before the announcement by OLC, a Pirates port was one of my predictions for their next major expansion. It's a logical and seamless fit for TDS, and I would be surprised if something similar to SDL's Treasure Cove wasn't proposed by Disney to OLC in their expansion discussions. I suspect it's still one of the frontrunners for the expansion pad besides Indy just because it's still one of the most popular IPs that can fit into TDS. OLC's recent stance has been to focus on exclusive additions though, so a potential Pirates port would have to be quite different from Shanghai's (similar to how TDS is getting a unique and larger scale Frozen E-ticket).