What kind of unlocks/secrets do you expect will be in Kingdom Hearts HD 2.8 Final Prologue? To use Kingdom Hearts HD 2.5 Remix as a guide, where there were two movies as a guide that provided hints toward the plot in the series, namely Kingdom Hearts DDD HD and Kingdom Hearts 3, I would expect KH 2.8 to do something similar.
As for Kingdom Hearts Dream Drop Distance, the original game had quite a few unlocks, if I remember correctly. In addition to the in-game theater and glossary, there was something called "Chronicles" that contained text summaries of the plots of every Kingdom Hearts game prior to it (all 6 since Chi hadn't been released at the time) that unlocked rather early on in the game after you cleared a couple of worlds. Another unlock in DDD was once you beat the game you could find a secret message in the game's credits that hinted towards the 1.5 and 2.5 HD Remixes being in development.
I'm a bit curious at to whether the Chronicles (and in what form) and the secret message will make it into 2.8 and what other unlocks and secrets await. I might also end with a question of whether we should expect a secret ending in Birth by Sleep Episode 0.2.