For the guys who have read all the books, how much more of the TV series could be created based on what's left in the books?
Certain story lines (Stannis, Brienne/Jamie, and Sansa in particular) are either way past where Dance of Dragons ended or have moved in a completely different direction.
The major story lines (Bran, Jon Snow, Daenerys, Tyrion, Cersei) are either at the ends points of a Dance of Dragons or just a step beyond.
The only story lines that have not moved or have not been shown are the Iron Islands. I doubt Euron will make an appearance. In the show, I believe Balon is still alive, however he died fairly early in the books.
Will the next season require another book?
Undoubtedly, next season will cover most if not the entirety of the Winds of Winter. Winds hasn't been published yet, and who knows if GRRM is even writing. He's a legendary procrastinator. His publishers must be infuriated knowing they're going to lose a mountain of cash if HBO beats them to the punch.
What's the scuttlebutt on the next book? Will GRMM just skip the book and help the producer's go right to script for TV?
The scuttlebutt is that GRRM has had five years (2011 was when Dance was published) to get Winds of Winter done. There is ANOTHER book after Winds (Dream of Spring) that will finish the series. The pace that he is moving is glacier-like. I honestly think he's lost all desire and inspiration in the GoT world and would rather sit back and enjoy the fame and money. That's fine with me, except I wish he'd just be honest about it instead of dragging it along.
It seems to me that IF this show has an "ending", it will be with the entire kingdom unified under one leader on the iron throne. Given the number of groups going for it, that would seem to take more than one season. Though, Stannis Baratheon's situation fell apart pretty damned quickly.
I believe HBO has committed to a total of 7 seasons for GoT's. That would allow for two seasons to cover the material that will supposedly be in Winds of Winter and Dream of Spring.
Regarding Stannis, uh, like, wtf, that whole thing took at least a season or two to build up, then his entire army gone in a single episode. Though, he's not technically dead yet (did the big chick whack him?) and half is army is not defeated, they defected. And red is still around, albeit way north.
Dance of Dragons ended on cliff hangers for pretty much every character including Stannis. I'm not sure if Book-Stannis is going to end up in the same place as Show-Stannis. The HBO producers have had no problem deviating characters from the books (BRIENNE) to make the show more entertaining. I don't think GRRM particularly cares so long as the show ends up in the same general ending as the one he has in his head.