There are many PBEM games around, many set in their own fantasy worlds where the GM has done their own worldbuilding to create a world for their game, but there are many films and TV series which have already done the worldbuilding for you and provide a great setting for your PBEM game.
George Lucas has created a diverse fantasy world with spaceships and limitless types of aliens. It is popularly extended through many films, books and games. Creating a new scenario within this world would be easy, and because if it’s popularity you will have a large audience who are already familiar with the world.
Settings for this game could be a planet, a spaceship, or a group of people travelling the galaxy. There are also multiple time frames you can choose from when creating your game.
This is a rich science fiction universe described in all of the Star Trek series’. Each spinoff series has focussed on exploration so there are many locations already described, and it is not too hard to invent many more for new adventures.
The classic way to create a Star Trek PBEM is to think up a name for your ship, set up a goal (usually exploration of new alien worlds) and populate your ship with members as your crew. Because of Star Trek’s popularity, you will have a large audience already familiar with the world and technology within.
The TV series’ episodic format can be easily converted into a PBEM format – discover a planet, perform a mission, then leave to planet and go onto the next adventure. It can be tough for a GM to keep coming up with new worldbuilding ideas for new planets however.