Reworking on a previous franchise has plenty of downsides unless they completely knock it out of the park. The bigger one to me is bringing in new fans to the console. They announce something like Fable, there will be people who roll their eyes no matter how the game looks, no matter how much different it is, no matter how much BETTER it is. They'll say "same old Xbox, same old IP" and move on. I guarantee we will see a lot of this with the rebooted franchises once they get announced, and it will be hard to fight against this type of speak.
Now, if these new talented teams were working on something brand new, that would shine a new light on the Xbox side of things. When people see good things on something NEW... that sends a better message.
I don't know, I think it will be a good thing to have people come and check out the Xbox side of things.