Seeing what Cox is doing with Lords of Shadow, this is how I think it should be tackled:
1. IGA(who already has the gist of the story), writes the treatment, which will act as a basic plot premise.
2. Cox will have his guys use that plot premise and add some meat to it(y'know, take IGA's story as the foundation and build up).
3. Get Mercury Steam to make the game.
4. With IGA acting as supervisor and Cox as producer, get Ayami Kojima on board as character designer, and perhaps a few composers into the mix(Michiru Yamane will be one, though I'd love Yuzo Koshiro to contribute some, as well).
I'd love that! THAT could make one truly epic Demon Castle War game!!