Even if we say bringing MU Gul'dan back is out...it didn't HAVE to be Gul'dan in that role. If WOD had taken place on Azeroth or soemwhere else in the main timeline it could have easily built up a new character into that position. There's no reason it HAD to be gul'dan who opened the portal other than fanservice because MU gul'dan went to the tomb of sargeras too.