As said, heavily depends on the type of guild... building from ground up, for true neutrality and convenience, it would be preferable somewhere in Whiterun...
Dont know what your plains are in style and such but from the little info we got here, I would suggest someting in the lines of retaking a fort and gaining control over it...
How about Fort Greymoor?
Plenty of space for improvements, well defended, center location, no big quests involved (except for Agnes), and being a fort it is plausible for any group to establish there... from farmers to warlords... from necromancers to templars... anything is fine... And you can simply add structures there or, even better, add some kind of quest to rebuild it slowly...