Define the base world were you expect the players to travel give it details and other things. You don't have to go beyond that the world outside of that map can be whatever you need. A new continent for explorers, a archipelago the remains of which are a civilization.
Give plenty of detail to the back-story but leave room for interpretation between player and Dm's. Maybe the Cerulean Empire was controlled by Dragonborn or maybe Halflings or even Humans, we do know it wasn't elves or Tieflings though. There are details left for the individual Dm to decide so a player cannot rules lawyer a situation.