Plot 2: The Struggle for Power

Each faction has gotten weak to some degree, but the heartless have lost their leader. She's no where to be found so the heartless now roam the different worlds. Only Hades has power over the heartless but only in Olympus Coliseum. Xemnas has decided to rebuild kingdom hearts but instead of spending the time to collect all of the hearts, his goal is to collect the princesses of heart. Sora's keyblade has realized that trouble has arisen again, and has returned back to him. He is now able to defend the princesses and his friends will help in the process.

Sora plays all disney protagonists
Hades is currently in control of the heartless at its weakened state
Hades can control those darkness in their hearts