Jesse McCartney, the voice actor behind Kingdom Hearts's Roxas, is working on a new game in the series, if you believe his Instagram account.
Kingdom Hearts 1.5 is a PS3 "remix" that includes high-def versions of both the first Kingdom Hearts and Kingdom Hearts Re: Chain of Memories. It also has videos from the DS game Kingdom Hearts 358/2—the scene McCartney instagrammed appears to be from those. If the videos are getting English voice acting, the game's probably coming here.
Presumably McCartney isn't talking about Kingdom Hearts 3, which hasn't been officially announced for any platform yet, or some sort of other unannounced title in the series. Presumably he also isn't just making things up to tease his fans, because that would be really mean.
I've reached out to Square Enix for confirmation and will update should they get back to us.
For all you "Kingdom Hearts" Fans. Recording the next chapter! [Instagram via NeoGAF]