They are all characters from 'the real world' who, for various reasons, find themselves in a strange new world.
It is a trope that is used a lot in fiction, but not one that is used often in roleplaying games (the only one I can think of offhand is 'Earthdawn').
The problem with well know roleplaying games (Star Wars, Krynn, WoW) is that the players often know as much, if not more than the gamemaster. The problem with homebrew worlds is that the players have to know something, which can result in an enthusiastic gamemaster infodumping for an hour, or prepping pdfs, or wikis that no one really reads, anyway.
The way around it is the Stranger in a Strange Land method. Let the players make a character from a system they are familiar with and then (via wormhole, time travel, magic hubcap etc) have them transported into a strange new world. (I did something similar once where I lead a party of D&D characters through a witchgate into Talislanta). I really like this idea and am going to work on formalizing it in Extinction Level Event.
*In Order: John Carter of Mars, Miles Monroe of 'Sleeper', the children of “The Lion, The Witch and the Wardrobe’, Joel of ‘MST3K’, Neo of ‘The Matrix’ and John Crichton of “Farscape’.