Sasuke will join the fight and when he saw they were fighting in a death match, Sasuke wont let Madara to kill Naruto assunming Madara can kill Naruto... He will save Naruto to kill by himself... Couse they made a promise to fight eachother again... This is what i think right but, depends on the fight against Madara they could strength their bonds to eachtoher and remember the old times... What do you think???? Maybe there wont be a Sasuke vs Naruto match or maybe it will be for taking the title of Hokage???