I can see it going either way. :/ On the one hand, story "ideas" are six a penny and what actually matters is what you do with said idea. The execution is what's important and only baby writers whine about people Stealing My Ideas OMG!!! Which means that a reasonable person, who's been around the writing block a few times, will also understand that if you write your own story based on a three-line plot bunny you submitted to a fest, you're not cheating them or stealing their ideas of how they wrote it or anything like that.

On the other hand, fan writers aren't always that reasonable when it comes to the sanctity of one's OMGIdeas!!! Look at the huge kerfuffle whenever the subject of remixing without explicit permission comes up.

And on the third hand, I could see even a reasonable writer, if they're a bit unsure of themself, feeling like anything you write yourself based on the idea you gave them is likely to "show them up," just because you're Amanuensis and everything you write rocks truckloads of socks. So even if there's not a copycat sort of feeling, there might well be a, "Well, why did I even bother if she was going to write that, which is so much better than mine?" feeling.

That looks like two possibilities of miffedness and/or hurt feelings and only one possibility of the person being perfectly cool with it.

I think it comes down to your own reading of the group and what you think is likely, knowing the people involved. For myself, I agree with persephone_blue that there shouldn't be any problems, that your version will be very different from anyone else's and more fic is always a goodness. But no matter that it should be okay, I've got a few alarm bells ringing about this sort of situation because if fans were always rational there wouldn't be any kerfuffles or wanking. :P

Angie