[quote=@Cyclone]
What I'd like to get your opinions on is whether I should take the time to figure out a plot and iron out the major details before making the interest check, or just make it as the vague idea that we have so far and see what the majority of interested people want. I see benefits to both sides.
[/quote]
The problem with fleshing out a plot in advance is that players may start to wander away from it, in which case you either have to step in to try and force them in the direction you want them to go, or start rewriting everything to take into account what's happening.

I'd still plan on playing as a Fairy captured by the Dark Mistresses and converted to their side though, in case you'd like to include that in the plot. :)