What would be the issues involved in "forking" the GFDL? that is, could there be two distinct future licenses that would satisfy the "or any later version" clause?
Because it's the person reusing the content that decides which version of the license to use, not the copyright holder.
Either way, this is just a hypothetical exercise. If we know exactly what we want if we could have anything, it would make it easier for us to make changes to the GFDL. It's always easier to do anything if you have a goal in mind.
Something tells me it may be too late for that kind of hypothetical exercise. It sounds like the licenses have already been worked out.