Young Robert or Rhaegar has never had actual significant page or screen time, so it would be cool and all, but I don't know where you'd fit that in a prequel. The only really big period between Robert's Rebellion and the War of the Five Kings is the Greyjoy Rebellion, and I reeeaaally doubt they would do that, because it would be so... weird. Telling the story that doesn't really help explain things to show-exclusive fans, other than how Theon is in Winterfell (which is explained in the main show), just so you can sprinkle in flashbacks of Robert's Rebellion, seems like a really bad idea.
