Pretty sure this has been denied a few times, would split up the community and make the server feel smaller, don't think it's necessary with how many players there are right now.
There's a reason why this suggestion has been declined a numerous amount of times and it's simple - Dan wants all his players in one world instead of separating players from one another by implementing your suggestion.
I disagree with this. I think the server/population is fine where it's at now. There is so many options if you're doing bossing, which you can use to avoid other users.