There are currently (at least) four shows in development to tell more tales in the Game of Thrones universe once the main series has finished - Game of Thrones "spin-offs" if you will. Just don't call them that around George R.R. Martin. He's not a fan of the term.
"For what it's worth, I don't especially like the term 'spin-off,' and I don't think it really applies to these new projects," Martin wrote on his LiveJournal (also yes, LiveJournal is still a thing!). "None of these new shows will be 'spinning off' from GOT in the traditional sense. We are not talking Joey or AfterMASH or even Frazier or Lou Grant, where characters from one show continue on to another. So all of you who were hoping for the further adventures of Hot Pie are doomed to disappointment."
Another group that will be feeling disappointed? Those hoping to see Robert's Rebellion, a series of major military and political events that more or less set much of the main plot into motion. "By the time I finish writing A Song Of Ice and Fire, you will know every important thing that happened in Robert's Rebellion," Martin wrote. "There would be no surprises or revelations left in such a show, just the acting out of conflicts whose resolutions you already know. That's not a story I want to tell just now; it would feel too much like a twice-told tale."
Martin also pointed out that it's unlikely we'll get a sudden smattering of GoT-universe shows all at once. "What we do have here is an order for four - now five - pilot scripts. How many pilots will be filmed, and how many series might come out of that, remains to be seen."
So to all of you fans worried about the quality of these in-development projects, those of you who want to see Robert's Rebellion, and those of you fearing drastic oversaturation, Martin has this to say: In order, don't worry about it, sorry but no, and probably not gonna happen.