Pretty much since it was seemingly confirmed Game of Thrones would come to a close after is eighth season, there's been talk of HBO considering a prequel series spinoff for the show. While the idea hasn't exactly taken a major step forward yet, it is at least still an ongoing concern for HBO -- and it looks like it stands a good chance of happening.
Robert Baratheon vs Rhaegar Targaryen, by artist Justin Sweet. Image used in The World of Ice and Fire, from Bantam Books.
HBO programming president Casey Bloys confirmed to Entertainment Weekly that talks are currently happening with A Song of Ice and Fire scribe Martin, and while a specific story or time period has yet to be hammered out, it would seem that at least both sides are building towards a spinoff actually happening:
I would say it's still kind of preliminary ongoing talks. There are areas we are exploring, but I wouldn't point to any one and say, 'This is what we're going to do.'
It doesn't tell us much, but it's the closest we've got to an HBO executive confirming that talks about a prequel show are happening, rather than just being open to the idea. Martin has elucidated in the past that he'd love a miniseries based on the story of Dunk and Egg, characters from his Hedge Knight spinoff novellas, but there is a lot of Westerosi history a prequel could uncover.
Like, a lot.
Whatever it ends up being, it's cool that we could be seeing more of the world of Ice and Fire on TV beyond the end of Game of Thrones.