But late 2015 would already give them 2,5 years of development (plus whatever preproduction stuff they already have accomplished before the announcement)! I'd say 2016 is the date if they are running behind. 2017 would be absolute disaster and SE would probably be the laughing stock of the generation again. I really don't think this will happen. I think they have been planning their next-gen strategy for a long while now. They have their FF13+14 disasters over with this year and FFXV and KH3 are full speed ahead as their flagship titles after that.
Edit: It is kinda funny that I think there is this ''pessimism'' around the expectations of this project. I think SE has this stigma now that makes people fear every project will take at least 3-5 years or whatever. It was the same when I talked with different people how KH3 would probably be announced at E3. Most said it couldn't be possible for whatever reason. Mostly it was because ''2.5 isn't announced yet'' like SE was this clumsy rhino who could do one project at a time and every project would take forever to get out. I'll call it right now: people will be very pleasantly surprised by SE output next generation if they compare it to what we got this gen. I believe we will get at least 2 big totally original Final Fantasy titles and 2 big Kingdom Hearts titles and that is not counting any of the possible spinoffs or XV sequels or whatever.
i'm just being a realist really
"In development" could mean anything from story pitches to game engine tests and idea pitches. Nomura said it that it was too early to announce the game and he basically just game us a sneak peak. I expect KH3 to enter full production sometime after XV is finished (I guess 2014 at the earliest by what we're seeing). After that I expect at least 2 years of development on the title, giving it a release window of late 2016-early 2017. And that's not counting in the shenanigans Square has accustomed us to this past gen. Also, let's hope Nomura doesn't decide to make another game before KH3 (TWEWY 2?)
To compare, Ready at Dawn's PS4 game has been in development since 2010.