I haven't seen anyone express a strong preference for an exchange-based ficathon yet, so I think you're safe in making it prompt-based. And mirroring the masterlist on both sites probably makes sense. I don't know how you'd do the trope-claiming post, though, if you plan on limiting the number of people who can writ each trope. (Which, I suppose, you don't necessarily have to do. There are lots of ways to write most of those tropes.)