I don’t know how you guarantee somebody that what they write won’t be retconned later. Even if the current Marvel made that arrangement in good conscience, and with every intent in keeping it, someday it will be different people sitting in these chairs, and who knows what will happen?
We don’t really go into projects intending to retcon them, and the best way for a book not to get retconned is for it to be so good that everything that comes afterwards is influenced by it. But that’s a lot to ask of any project.
So GRRM is always welcome to talk to us about stuff–he’s a Marvel fan going back to the earliest days, so we’d welcome his ideas and contributions. But I don’t know how you make that deal.