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.
Source: io9 – HBO Is Still Talking to George R.R. Martin About a Game of Thrones Prequel Series