
So, say you run an ongoing Star Wars game, using whichever rule system suits you. Things are going fine, when when of the players show up after a binge-watching of the Firefly TV series. She loves the idea of playing a bunch of smugglers transporting a fugitive like River Tam. The rest of the players want to keep the Star Wars game going, so do you ignore her request or upset the others by taking time off to run a game of the Serenity rpg? What if there was a third option?
Captain Mal Reynolds is a smuggler, right? Star Wars has those. Lawrence Dobson, Jubal Early and the nameless operative from the Serenity movie are bounty hunters? Star Wars has those. Serenity is a well loved but kind of beat up light cargo ship. So is the Millenium Falcon. Alliance, meet Empire. Browncoats? How about Rebels? Serenity has psychics with uncanny combat prowess. Jedi and Sith, anyone?
So what if the Star Wars PCs, 'freelance traders' with a Reynolds or Solo willingness to ignore the law and do whatever they have to to make a buck, pick up a strange new passenger who recently escaped her Sith master, who recruited her through lies and false promises after discovering her potential skill with the Force. This "River" is still a young padawan, not fully in control of her abilities, like proto-operative River in Firefly. The Empire wants her back, but has to keep it on the downlow to keep up the illusion that the Jedi and Sith are just folktales from an old religion. Almost every scenario involving River could be played out in the Star Wars universe with these tweaks.
"Captain, get a load of this frikkin' guy"
Obviously, some SciFi sungenres and brands would be harder to mix than the example above. William Gibson's dark, gritty cyberpunk tales are a little out of place in a Star Trek universe future earth setting, and the almost Groundhog Day futility of time travel of 12 Monkeys (the movie at least, I'm not really familiar with the TV spinoff series.) doesn't really jive with the Terminator 'verse, where time travel has predictably happier endings, at least until the next sequel.
On the other hand, Aliens style xenomorph bughunts would work great in a Starship Troopers game. Zombies, Resident Evil or I am Legend style (meaning, they're a chemical, biological or other "sciency" accident, not magical undead) could be added to the mix to create a really bizarre but fun twist on War of the Worlds, where the humans the aliens round up aren't outright killed, but comeback later as mindless cannibals.
I'll go into more detail on mixing ideas from various SciFi in the near future, specifically relating to how I use the somewhat generic setting of the Traveller RPG to host any manner of near to far future SciFi. That was, after all, the original intent of that game; to be a catch all for playing any SciFi game you could dream up. The allure of selling sourcebooks inevitably leads most games, Traveller included, to produce a default setting for game play, but Traveller, intentionally or not, left things open and adaptable enough to easily plug all kinds of stuff in without destroying the setting.
For now though, what's your take on mixing settings? Are you a "Reese's" GM, or do you prefer your chocolate bars separate from your peanut butter sandwiches? Let me know in the comments, and tell us about your experiences with mixing stuff together.

Along with being the current licensee for a 
Another reposted guest post I wrote for