After cancelling the plans of a prequel set 5000 years before GoT, HBO has announced that they have given another prequel a full series order (10 episodes). This one is based on George RR Martin's book about the Targaryens time on the throne. It's set 300 years before GoT.
https://ew.com/tv/2019/10/29/game-of-thrones-targaryen-prequel-ordered-hbo/
https://ew.com/tv/2019/10/29/game-of-thrones-targaryen-prequel-ordered-hbo/