I think if it ends up being Sasuke and Naruto at the end Naruto will win so there needs to be some sort of very strong third person to come in in the middle of their battle, maybe Orochimaru or Itachi to lay down the law on either or both of them. That way either Sasuke will be saved by Orochimaru or whoever comes and continue his quest of evil to become stronger or naruto and sasuke put aside differences to fight together (leaving a reason for him to come back to konoha).