This is a fantasy world with magic and multiple races. I'm resisting a technological campaign because I think it's easy to get lost in the minutiae of the environment.
The world gets incursions from other realities/planes/multiverse. These incursions bring in new races and cultures. This introduces the option to bring in alternate histories like an evolved Roman legion fighting Mongol horse archers riding Pegasi. This is also a source of conflict in the campaign.
Almost all creatures have some magic. In humans this means most everyone has a magic power. For many people it's useless (turning your hair green) but some have beneficial abilities.
There are many species that have entered the world through incursions, some of whom have yet to be discovered. This is a world where you can be almost any character you can imagine!