Does anyone dabble in this? I have been considering a couple of treatments, one on characters in TV show The Big Bang Theory, and a couple of stories around the recent TV show Ghosts. The Ghosts one regards current events and would play out like a satirical skit on a show like SNL.
Oh yeah, we've got a few writers doing fanfic. I don't remember who offhand but several. I know Wreybies, who used to be the admin and was a moderator when I joined, was big on fanfic.
I'm supposed to be working on a Starcraft fanfic, but I have a case of the lazies and am blaming it on my health stuff. Granted, I've done fanfiction for years, and have a five part Mega Man fanfic that I'm actually decently proud of.
Hmm ... I once wrote a five-part fanfic, celebrating the adventures of a hero who was doing his best to kick the alcohol habit, and based on a very-old-but-oh-so-good computer game called Quest for Glory. Do yourself a favour and look it up. It's fantastic. (The game, that is. My fanfic? Hmmmmmmm ... it's not bad, but it's a bit ... meh. But then, it's been about 20 years since I wrote it. My writing skills have sharpened a lot since then). I also wrote a 13-chapter fanfic, where Leisure Suit Larry (does anyone know about him?) comes to the Quest for Glory universe, becomes king, and has to deal with political issues, despite being a lecherous so-and-so who's obsessed with sex and finding "Miss Right" (but often settling for "Miss Right Now").
I've heard of Quest for Glory. A youtuber called One Short Eye did speed run videos on a couple of the games. Me personally, the Sierra games I'm more into are the Space Quest ones. Because, you know, space is the place. I wish I knew who owned the rights to that game so I could nag him to make another. If you're talking about what I played, I mostly went for Zerg, before moving on from gaming in general. I would have tried Terran if I didn't have a thing called real life to account for. As far as fanfiction goes, I prefer Protoss. They have the best story, even if I suck at actually playing them. My favorite character is Aldaris. I love his complexity.
The Space Quest games are nowadays owned by Vivendi. You could try Vivendi, but I wouldn't hold your breath. Having said that, the Two Guys from Andromeda -- you know, the ones who made Space Quest -- are back, and released a new game. Space Venture! Check it out.
If nothing else, fanfiction is a good writing exercise. Generally speaking it covers all areas of story, less world building. That is assuming your are using original characters in the world. Any exercise that helps you advance your craft is worth doing.
True, you won't learn much about world-building from fan-fiction. (Also, character development; most characters in fanfic already exist, whether in a computer game or whatever). But sometimes, characters in a computer game aren't fully fleshed out. Here's your opportunity. When I wrote fanfic, I created a hero who saves the world, etc. hooray ... but is also battling alcoholism. (No, he's not a hopeless falling-down-in-a-ditch drunk, or else -- let's face it -- he wouldn't be able to save the world. He just likes a drink at the most unlikely times, and -- not often, but sometimes -- his possession of a hipflask helps him save the day. Huzzah!) So, yep: this fanfic was definitely silly. But in a good way.
Many fan fiction stories I have read avoid the MCs, and focus on a secondary character, one who is mentioned in passing, or amounts to guard #2. The other approach is to introduce all new characters set in the world. The second was the approach I used when dabbling in the world of the 1631 series. The series MCs got a passing reference at best.
In the ones I've read, and written, for Harry Potter they focus on pairings. I love reading Hermione and Narcissa or Hermione and Bellatrix. And, yes, they can contain sexual scenes too. I have never read, or written, any for a game.
I write fanfic for fun a lot of the time. I find it my 'fun' writing when my original stuff is stalled. I love exploring different characters and being unreasonably mean to them. (Because I can and who doesn't like a good angst fic?) I am currently tossing around a fanfic for Dark Souls 1 (and indirectly Dark Souls 3) and have been having fun. I think building onto the lore and world is really fun and characters expand as I write them. Although, I have to admit, I tend to not write stories about the main characters, but the villains in general. So, I get a lot of fun out of developing the villains in a fandom. So many headcanons. As for pairings, I have a few I like, but nothing is set in stone for me. I don't tend to write romances and when included are casually there. I personally don't try to force romance on any of my characters as the plot is more important to me.