I agree with you all, and the only game i think it could be in is 358/2 days. Think of it, you could play as Xemnas and fight against Xehanort's Heartless.
And wouldn't it be cool if Roxas was the one who stopped Xemnas from reclaiming his heart and that's why he got kicked out of the org. and labled a traitor?
You must have played KH2 and didn't pay attention to it. He is labeled as a traitor because HE LEFT THEM, to go find Sora. And 368/2 Days takes place BEFORE KH2, and therefore a fight between Xemnas and "Ansem" is unlikely, seeing as how "Ansem" is destroyed.