For games like that, infinite world is a problem because the bigger "visible" world, the bigger performance issue and developers can't predict how far players go. Other problem is traveling and transportation on big distances.
I think that better choice would be looped world like planets. In that way you don't have borders and still maintain fixed size of the playable area. Adding layers (underground, next planets) can expand playable area to enormous size and add different environments without worrying about smooth transitions between them.