Nerd Kingdom's The Untitled Game (TUG), the focus is on complete freedom. One of those freedoms is the ability for anyone to make his or her own server in the form of a floating island with any rules.
Official servers are broken into two categories: Survival, where your items are not safe, and Adventure, which follows more of a traditional RPG/MMO ruleset. But in the private servers, your floating islands will be yours to do with as you please.
TUG is currently still in its Kickstarter campaign with 9 days and around $75,000 to go before the project is funded. Head on over to its Kickstarter update page to learn more about private islands and the other features found in this sandbox game.