It would certainly be nice to see areas change and new NPCs move into areas you've cleared out, but it's quite a lot of work for something most people probably wouldn't even notice or be bothered to deal with. Some people would feel compelled to return to every place after a while just to see if something has changed, when most places probably wouldn't have. I'd file it under the ideal, but probably not happening to any meaningful scale.
I think being able to go back to Fort Joy is unlikely, but I could be wrong. You're trying to escape from the magisters, so going back to one of their places would be very dangerous. I don't think the teleporting statues necessarily would have the power to send you across huge expanses of land, considering they've already removed their power to teleport you anytime like in D:OS1. After all, it seems like if a prisoner was attuned to a statue from another land, they could just teleport away from the prison once they got their (although, that assumes that everyone has the power to use these statues, which probably isn't the case.)
To me, there's something about gating off old areas (with good reason) that actually makes moving forward more exciting. There's more of a thrill to finding merchants instead of just going back to the old ones and there's a sense of isolation to your characters, like they're really on their own. And really it gives a sense of consequence, like you've left everyone else on the island, and you can't just come back and deal with their problems on your own timeframe. In a way, it makes it unnecessary to actually program changes on the island, because it can just leave room for your imagination of how it has changed.
