Well. No idea what the dev team is thinking since we have no hints that I know of, whereas when heavensward was coming out they mentioned they considered doing swimming then, but decided to do flying first.

Personally, I'd just like to see swimming expanded on a bit. SB didn't really utilize it that much, which I guess is good in the sense that it didn't wear out its welcome. But it seems like it could be utilized a bit more without becoming obnoxious...