That's what I also expect for! Like... Utada has been, for me, a really important part of this game! You know, as if "Simple And Clean" directly brought you to your memories of the game! Like the good moments you had playing it, and so on. I might talk for myself, because I experienced that but I know some people probably are in the same case! The day Kindgom Heart II was released and I heard that on TV, I could swiftly hear parts of the song and I went crazy because I couldn't hear the whole "Sanctuary" song, and I was like Writing the song and Imagining how it could be like, it could sound like (now knowing the result, it was totally wrong! lol). But Utada is important, on this game. That wouldn't have been the same thing if Simple And Clean wasn't the theme of "Kindgom Heart Birth By Sleep", Kindgom Heart Chain Of Memories, Kindgom Hearts 358/2 days, and others I never played (like Coded and Final Mix, etc.)! xD
If there has been another artist for the following opuses of the game, that wouldn't make me sad if she didn't make it on the III. But there again, it shows, if there's Utada, 'cause if she's not there, that is really going to feel awkward to me!