Simply focus on adding a new world and call it "The open world" this world will simply be an online session for players all over the world to play with each other, so whoever wanna play with real people online they simply sail to the open world where they can find other players to play with or against, the simple concept as any new world DLC, we sail to open world, we take over an island "colonize it" then build on it, then we start and rush building our islands to play together, form alliances with each other or fight, using our resources from our regular islands from the new world, old world, cape, enbessa or arctic.
if you did this for your next DLC, i bet you ubisoft 100k that you'll sell at least 100k copies of the game. Also try to add some stuff to make the game spicier since we will be playing online so we will need more strategic weapons, defenses, and tactics but if not you can leave it as is, just the open world will do.