Well, seeing as Utada just recently made the theme song for Evangelion 3.0 (is that correct? I'm not a fan of that anime, really), she stated that Evangelion holds a special place in her heart... I remember her saying something to the extent of, "I just HAD to make the theme song for it!"

I'm not sure if Utada has stated in the past that Kingdom Hearts holds that same special place in her heart, but I've never heard her talk about Kingdom Hearts very much, which worries me. I'm not sure if the creators of Kingdom Hearts had Utada on a contract for a specific amount of time with which they could use her to create music for them... or whether that contract expired by now (if it was a type of contract at all). If her contract with Disney DID expire, since Utada is on hiatus from regularly making new music, I doubt she will make new music for Kingdom Hearts 3. But I'm sure they will request--or have already requested--her to make a new theme song as both of the previous themes were mega-popular and greatly benefited them both.

Time will tell (pun intended), because the trailer they showed off for KH3 was one that they admitted was probably way too early to show the public. Hence, they recycled the Passion/Sanctuary theme in the meantime, which I'm sure is a placeholder for the new music (derp). It's Disney, so i'm like 95% sure they will have vocalized music... and it won't be the same without Utada. But... in the end, I'm sure they will make it a business decision and make the easiest choice that will gain them a quick buck. Who knows?

EDIT: Could have possibly been a contract with Square-Enix... but then again, I don't know how that sort of thing really works...