I'd buy the Black Company magic system all by itself easily. What I'd really like to see though is a book that contains the magic system, the company/army combat system, and few other subsystems for playing a variant of Pathfinder that has the gritty low magic* war heavy feel. The specific setting is something I like and appreciate but the feel you were able to make with the original RPG book could easily be developed into a standalone tone and advice book ala Heroes of Horror.
I nominate:
Book of Fiends
Plot & Poison/Advanced Race Codex: Drow
The Freeport Trilogy
A True20 equivalent for Pathfinder would be pretty neat. Heck, why not a Blue Rose revival?
Book of Fiends
Book of the Righteous
Plot and Poison
Honestly, I would prefer them to release those products without kickstarting them. They have excellent material, both old and new, but they fail at stuff like meeting deadlines or communicating to backers.
I'd like a PFRPG version of the Book of Fiends, but at this point, I'm extremely hesistant to become a backer on a GR kickstarter again.
Not sure where this is coming from. I backed both the Advanced Bestiary and Freeport and I get update emails from Chris Pramas all the time. He's been incredibly transparent with the timelines for both books, and I'm very happy with how those Kickstarters have turned out.
Ok, here's my list, in order.
1. All of the Freeport books, consolidated and expanded, with full color artwork.
2. The Book of Fiends, expanded with full color artwork.
3.MUTANTS AND MASTERMINDS FOR PATHFINDER!!!