I wanted to see Robert's Rebellion. That ties in so closely to the events of GoT and can keep so much character familiarity that it would most easily maintain fan interest. But I'll give GRRM credit for such a rich backstory. They can set a prequel anywhere in the timeline from the Age of Heroes onward and still have a compelling series. So if they back to The Long Night Era that gives them plenty of room. If if stays popular they can progress all the way through the Age of Valyria, the rise the Seven Kingdoms and the Targaryen Dynasty and eventually get to Roberts Rebellion.