If this is to be the first of a possible trilogy, the only thing I'd worry about is Ridley Scott. Sorry to be ageist, but he's 74 now. Isn't he doing Blade Runner 2 next, or at some point at least. Thus I can't see Prometheus 2 coming out till 2016 or so. And then to do a possible third one as well.
Ha I'm not saying he's gonna drop dead at any minute, but I would just hope they got completed is all