Modular Road System
Every once in a while a builder posts something so clever or well done it could be the foundation of a real line from LEGO. This modular road system by builder LEGO 7 could absolutely be developed into a full-fledged theme. I could even see it being a sub-theme of established properties, like Star Wars or Ninjago. It's practically there already, with a cameo by Optimus Prime and Bumblebee.
Original author: Tommy