I have a character who is from the Shadowrun universe, and brought all of his gear into the PPC with him (Most of which, aside from maybe the armored jacket and the rather obvious cybernetic arm, will likely never see real use, especially not in a mission), as well as knowledge of his particular version of the future.
For those not aware, Shadowrun is basically Earth, roughly 60 years in the future, and with corporations ruling almost everything. Well, plus magic. So, for example, for him something like Harry Potter is classic children's fantasy, much the way the Chronicles of Narnia are for us. His knowledge of videogames is largely incorrect, because most of today's games are either part of history which no one really touches outside of maybe a "history of gaming" class at some university, or they're part of a series that's still ongoing, like say Zelda or Sonic, but most of the canon he knows for those is stuff that hasn't happened yet, and may never happen. Legend of Zelda: Lamentations of the Flame Princess may never happen, for example. (In fact, I'd be surprised if it did, since that's currently the title of a D&D adventure module, minus the "Legend of Zelda" bit.) He knows a lot about film history and "classic" comics, but he thinks about movie and comics from today much like we would think of stuff from the 60s.
As for fics based on RPGs, almost any RPG will have canon characters. Games popular enough, like D&D and Shadowrun and World of Darkness, tend to also have novels based on the game settings, which give you an idea of what the canon characters are like. Even if there are no canon characters in the story, if you know the world well enough, you should still be able to spot traits that are Sueish, or non-canonical. Elves in Shadowrun? Sure! Immortal Elves in Shadowrun? I'm side-eyeing you pretty hard unless they're an established canon character. Elves of any stripe in World of Darkness? No sir, not on my watch. Similarly, demons in D&D or World of Darkness? Sure! Real demons in Shadowrun? Uh, no, not likely. Guns in Shadowrun or World of Darkness? Absolutely, it'd be odd not to see guns in fact. Guns in D&D? It better be like a blunderbuss, and even then they're extremely rare.