I understand the story has been told, but hey, basically all the backstory of Westeros has been told so unless they come up with original ideas, people will already know the details.
I'm all for a prequel about Robert's Rebellion and I think it would connect direcrly into the events of Game of Thrones - forgetting for a moment that there was an actual book about it, the prequel for Lord of the Rings was The Hobbit, which had Bilbo and Gandalf in it and other characters people knew, it was not a random story from the Silmarillion.
About the books... gut feeling is that, in 2-3 years, we will eventually get The Winds of Winter. A Dream of Spring is unlikely to ever be finished.