I thought he might be the author of Henry's book, but I also like the theory that he will be involved with Anton's beanstalk in some way because of his name. Maybe he'll be a part of both stories?
Even if Cora is dispatched this season, which I'm fairly confident she will be, I think that there will still be threats involving other nefarious characters getting a hold of the beans, so maybe that's why Greg and possibly HER will be around next season. Maybe Greg's a harmless enough person now, aside from knowing that magic exists if he really is just a regular person, but the thought of gaining access to and then control of the beans, could cause him to go dark and into villain territory.
Ok, I really like this Aunt Bee. Because I agree. Cora most likely will be gone at the end of this season, but we still need to have a baddie for the next season and having a magic bean stalk that can take you to other realms ALSO means that other can come here, thus ensuring more evil villans.