Bold half-plausible/half-absurd prediction: 3.5 Remix with DDDHD(FM), KH3 "Prologue" demo, & Chi Theater Mode.
Dream Drop Distance Final Mix
-Its a given we'll get DDD in HD, but the long shot is it being a FM version. On one hand, it wouldn't be right if DDD didn't get the update the other main entries had & there's definitely still things that they could add. On the other hand they probably wouldn't have the staff to spare on developing new content (maybe the theatrhythm crew?).
KH3 Demo/Prologue
-I've said before that bundling DDD with KH3 would be pointless because everyone will just play KH3. But DDD by itself would seem like a downgrade to consumers compared to the collections. What to do in lieu of another full game? Why a demo of KH3 of course! Even if short, it would be 100% new content instead of a rerelease so that may balance out.
Long shot: The demo is actually standalone content that could pass as its own game to an extent, a la Ground Zeroes, Dead Rising 2 Case Zero, or Dissidia Prologus. This way it would fullfill the 2 games & 1 movie quota by technicality. It could even be beneficial to KH3: No one wants a repeat of how KH2 started, so splitting a lengthy opening world off as a glorified demo would be win/win.
Of course by putting such a title on the disc the remix pretty much has to be PS4/XB1 & not PS3 by default. But really PS3's time has past & many want the other 2 remixes ported to the new systems. Might as skip having to port 3.5 as well.
Kingdom Hearts X Theater Mode
This is happening, period. Oh sure, they didn't show any KHX in those end credits but then there isn't exactly anything to even show at this moment so that probably doesn't count. If KHX's story is so essential & they still aren't localizing the game, this is pretty much the only option, ain't it?