If Itachi remained a villain and fought alongside Obtio and Madara.
His reason for doing this- "I just want to enjoy watching the world burn like I enjoyed killing my pathetic weak clansmen"
Sasuke joining the battle with Naruto would have added new meaning this way.
Sasuke could have fought Itachi now to avenge his previous loss at the Uchiha hideout, would have been epic
If would have made both Naruto and Sasuke look good.