By setting the new show ahead of the original trilogy, Star Wars has opened the door for other Star Wars TV characters to appear. The franchise has already produced two shows set before A New Hope, with survivors who could appear on Cassian's new show. The rebels of Rebels could even make sense, depending on the precise timeline! Hera was involved with the Rebellion for years, even before Kanan knew that they were part of a larger group of rebels. Let's see some Rebels rebels in live-action!
Source : https://www.cinemablend.com/television/2461073/star-wars-next-live-action-series-will-be-a-rogue-one-prequel