Yes. Mickey-Xehanort. Think about it. He could be the final boss (or a regular boss) at the end of KHIII.
It's pretty simple. Sora, Riku, and Mickey defeat Xehanort once and for all.
Or so they think.
As Xehanort fades away into Darkness, his heart is exposed, while Sora and Riku turn away to go back to their homes. Mickey looks at Xehanort's heart. The king starts talking to the dying Xehanort about the mysteries of the heart, and Kingdom Hearts. He lets his guard down, long enough for Xehanort to transfer his heart into Mickey's body.
After becoming Mickey-Xehanort, he knocks Sora and Riku out cold, who doesn't know that Xehanort has one final vessel. He summons the X-blade to finally open up Kingdom Hearts, and begin a new keyblade war.
Read more >