Yeah, the prequel is going to be set hundreds (maybe thousands?) of years before the GoT series during the Age of Heroes supposedly. This is when the first Long Night happened, and I believe pre-Doom of Valeria too. From the articles I've read, there does seem to be some hints that the Valerians/dragons will be included - I think during this timeframe they were still conquering in Essos? I don't know, I haven't read the wikis in awhile lol. I do know this is when Bran the Builder put up the Wall.