Removing the ability to teleport across the sea (Hearthing, Thingwell, charterstone, ect).
Players who want to flee the main island and hideout thought the world, don't want to be visited every 10 mines by another player walking by looking for pvp or hunting their areas.
Limiting the players from crossing the sea would tax those who do with time, needing to use a boat to get on and off the island or to be KO'ed to be sent back to their fire.
If a group of players invade another island, they can't simply hide and teleport back inside their safegates that they erected a day before hand.
For trade this would make it a little more localized, and shipping items from island would be necessary; but I doubt it will have a large impact.