I'm a bit rushed for time, but I don't want to forget having read this and not responding. Plus, I feel like this sub-forum doesn't really get enough love/attention. Games here are meant to be discussed, critiqued, etc. Otherwise, it's no better than the advertising section. :/
So, good on you, Strude, to offer ideas! Haha, look at me, praising people like people actually care about my approval, lol.
I have a few points I wanted to raise:
1. While the open-world/free-roam idea is attractive, I've yet to see it work on WTF. There's nothing inherently wrong with the idea, but WTF may not be the audience. A lot of players here seem to prefer a story (it doesn't have to be anything big or grand. It just has to be a motivating point) and stories are excellent GM tools to nudge players into activity. Like Strude said, having a loose cast may be useful because a set cast means definite diversity of characters, etc.
2. Strude touched up on this, but I figured it's worth elaborating on: Games like these are often "high-skill cap." What I'm trying to say is: You'll need a group of writers who aren't a herd of sheep (aka people who can't be active in their posts and constantly rely on other people, namely the GM, to guide them). If you don't want a story, it's imperative that you gather active writers, who can bounce ideas off of each other. But I could be wrong about this. I'm not sure how you ran Eternal Sands and you may have found the balance for setting up such a game.
So, basically, I rehashed what Strude said, but I just wanted to share my thoughts as well. I know this sounds really negative, two people saying the idea may not float, but it's not. It's just an observation of what has worked and what hasn't over the past few months. If you could link to your previous game, Eternal Sands, it might help people to better understand how you had run things originally. (: