Backstory: The repercussions of Robert's Rebellion are what has driven everything we've seen in Westeros over the last six seasons. After centuries of Targaryen rule, heir to the throne Rhaegar Targayren falls for the wrong girl and in doing so tears apart the entire kingdom.
When: 17 years before Game of Thrones.
Connections: With it being so recent, nearly every big player in this story is someone we've seen before. Ned, Robert, Lyanna, Tywin, Jamie, the Mountain, Jon Arryn, Varys, Pycelle, Cersei – not to mention baby Jon and baby Dany.
Pros: The advantages of making a show about Robert's Rebellion are clear. Within the Song of Ice and Fire series, George R.R. Martin has given us a comprehensive account of what went down, and having so many familiar faces – even if they were played by younger actors – gives continuity that very few other spinoffs could. Fans have been waiting to see events like the Battle of the Trident or the tourney at Harrenhal played out on screen for two decades.
Cons: The rebellion being so tied into Thrones means it's something fans would love to see – but it's probably the very reason that it won't happen. Ever since Bran became BFFs with the Three-Eyed Raven, we haven't needed a prequel to see what happened back then. We've seen the Tower of Joy, we know that Jon Snow isn't Ned's son, we've even seen flashes of the Mad King yelling "BURN THEM ALL!" as Jamie shoved a sword in his back. By the end of Season 8, who's to say we won't have seen the rest of it?
Robert's Rebellion could one day make a great movie, but when looking for a new show you'd imagine that the showrunners will want to start with a clean slate.