Video Game Fanfiction

WARNING: Topic may contain spoilers, and examples of good vs. badly written fanfiction. Proceed with caution.

Hi there. I had a scary moment today, and it led to a question about how to write good video game fanfiction. I'm used to writing anime fanfiction and short stories, and to me, games don't always cross media very well. But I had a moment today where I wanted to write it.

See, recently, I started playing Portal 2. And last night, I finally got to the Tartaros testchambers. And all the stuff between me and G La DOS before we got there was fun. And finally getting to pull the biggest troll move ever on "The Queen of Lies" (my pet name for her). But through all this, I actually started to feel BAD for that bitch. I know she's getting what she deserves

But this morning, when I woke up, something happened that I never thought would happen.

...I had an idea for a Portal hurt/comfort fanfiction.

I refuse to go into specific details out of modest amounts of shame, but I'll say this: Good fanfiction does exist. But for a lot of games, it's hard to write. So, tell me, guys: what criteria do you use for good video game fanfiction?

Do not just retell the game with a minor name change or just slightly different choices. Add something to it. Serious character insight, more dialogue between characters, alternate solutions to problems the game didn't let you do. Also, you do not have to do every little detail just like the game. Gameplay and Story segregation and all that.