I’d love to have a prequel series all about Robert’s rebellion, as the events of that particular civil war directly led to the events of Game of Thrones.
That said, many characters from Game of Thrones had parts to play in Robert’s rebellion.
They are all prequels, and Casey Bloys has said that there will be no existing characters from Game of Thrones branching off to new shows.
It sounds like the prequels may stretch further back than simply one generation before the War of the Five Kings.