my 0.02:
Writing is great fun, just dive in and do it. If you find the idea of a whole game's worth a bit unnerving, start with some short stories (a few pages or so) and work your way up.
All my writing so far has been SE4 based, but not related to any actual game. I've just started some game-based writing and I have to say it's pretty daunting. I prefer the freedom of not being limited by game events, game maps etc.
Whether or not you're using an actual game as a guide though, I think the most important thing is to put some thought into your characters and their setting.
Always be asking "what would my character really do in this situation?" If you don't know, then you need to think harder about your character. It's a great way of building the characters up and avoiding unbelievable/ unrealistic scenarios.
If you have believable characters in an interesting universe (one with lots of scope for exploration, conflict and general scifi wierdery) then the story will simply write itself. Your head will fill up with great events and your only problem will catching them all on screen before they get forgotten again=-)
Finally, get as many ppl to read it as you can. Not just us lot here, but also your friends, family, colleagues... anyone who can give you an opinion and suggest improvement. Try the
http://www.speculativevision.com/forum/ forum, it's a place dedicated to offereing constructive criticism to authors.
The wider the spectrum of readers, the broader your universe will become.
On that note:
Have you read Othaglot and Cane? Read it today! Just click the banner below for an interstellar tale of murder, betrayal, intrigue and a gratuitous alien shower scene.