Something a little new?

Ah, the forum-adventure style of story telling. It's been done before, but not so much for fics (which ideally don't involve screenshots... and if you are basing a story straight off a game you'd want to be clever in doing something different to just being the game in written format, because if it's that why read it when you can play it?) As such if you actually want to do a fic with the plot/events based on user input I'd suggest given written scenerios rather than screenshots. (Or you could do drawings instead, but I'd think that'd fit better in Fanart as an interactive comic).

Another problem to consider is the user responses themselves - sometimes they could lead to a position that's hard to write 'out of' to get the story back on track, or just be plain stupid (see earlier mspa comics for instances of this where Hussie, iirc, took the first suggestion for each page). As such you'd need to strike the balance between having control over the story and still letting readers have a say.

Initial thoughts that sprang to mind anyways... hope they're of help. I'd think is doable and could be interesting, but it won't be an easy project by any means either. Checking out Role Playing games may be an idea too as they are similar (but rather than having users say what should happen, they write as characters in the story and so determine what happens with their own creations, with a story owner keep everything in check and moving, and controlling the main events of the story).