In addition to getting all three Fable games, it is our understanding that the DLC content for the Fable games will be included as part of the package. So, those just starting to play Fable will get the entire Fable experience, including the DLC content, which adds a few interesting things along the way that we can think of.
While the release of the trilogy together in one package does not apparently signal the end of the Fable franchise according to sources, it is clear that they want to get the Fable Anniversary HD remake out there and see how well it does before they start thinking about the future of the franchise. No word yet if that future will include the Xbox One; but it does seem logical that the next step would be an Xbox One release. So far, no official comment has been made.