I used MCA Selector to seamlessly stitch together 15+ survival (!) worlds together, for me and my friends to enjoy. What seemed an easy technical challenge at first proved to be a somewhat difficult game design challenge; not all 20 players on my servers wanted their house to shift away from the ocean or have a jungle next to their hobbit hole. 

Eventually I settled on people and their cities remaining somewhat distant, but giving them an opportunity to "build" teleports like boats. This let people keep their surrounding worlds but play together easily! 

I've been playing this game for over 13 years, during which I came to realize that Minecraft is not just a game, but a way to express myself and celebrate my friendships. This is why I consider this world as one of my most important creations.