[HELP] survival game

hi everyone. im starting to gather a team and work on a survival sandbox game together. in this kind of game, you can build your vehicle using the materials in the world. then you ride it, explore new places, get more materials, build more stuff for your vehicle, stuff like that. guns, rooms, decoration, something like terra tech but with more advanced programming.
BUT i need absolute help with one question: which one should be the world? should it be on land? underwater? in space? what exactly? and should be modern or futuristic? what makes this game more popular?
thanks if you post a comment about this.