I know Xion was made from Sora's memories and people think that she can't be brought back. But I think there are still memories OF her. Such as Roxas eating ice cream with her. And I know that he could not remember her anymore, but maybe inside him, there is one memory of her stored inside him. But now Roxas became one with Sora. Maybe if Sora could have touched the Oblivion keyblade, Xion could be remembered like Riku did. Sora could remember her by Roxas' memories that might be stored inside Sora's heart