I don’t think they will do that because it would have him senior to Admiral Mullen the Joint Chief of Staff. Maybe if they promote both of them it would work. I don’t believe that they will have anyone senior to Admiral Mullen.
Commander, U.S. Forces Afghanistan is subordinate to Commander, U.S. Central Command, and then to the Secretary of Defense. The Unified Combatant Command chain of command does not run through the Chairman of the JCS.
I do not know if Commander, ISAF is subordinate to the NATO Allied Commander, Operations.
If Congress created an exception were created to Goldwater-Nichols, a position of Supreme Commander, ISAF could be created which reported directly to the President and the Secretary General of NATO. This could justify five-star rank.
I actually believe such an action is warranted if we view the war against Islamic extremism is an existential war, and not simply a regional theater battle.