You folks at Rare have said that a world expansion within the same map isn't possible, which is due to lag. If so, why not use the portals that were used in Pirate's Life or something similar to travel to another map? Like, multiple Seas of Thieves.
Another idea would be to unlock it with a device on the front of your ship (or anywhere, doesn't matter) by inputting some sort of treasure item in it, similar to having our very own Shroudbreaker.
The reason I think this could work is because each map could be on a separate server, as opposed to it just being one massive map bogging down one server. Plus, it solves the issue of players being spread out farther, so I don't see why it wouldn't be too hard. :)