Utada Hikaru tweeted that she saw Kingdom Hearts 2.8 trailer, which means she must know where this is leading.
Some years ago, her father said on Twitter she would return to work in KH3, but that turned out to be a misunderstanding (or maybe not), and he claimed it was still undecided afterwards. You can read that story here: http://kh13.com/news/43722-utada-hikaru-is-involved-in-the-theme-song-for-kingdom-hearts-iii
Her return for KH3 is still speculation nowadays, even if there's some hints here and there.
She tweeted about it twice:
https://twitter.com/utadahikaru/status/776303393069555712 (Japanese)
https://twitter.com/utadahikaru/status/776303791142477824 (English)
I still didn't thought she would comment something like this. The chances of her return are getting higher!