A YouTube reviewer called Angry Joe will be starting with KH3 and has said that if he doesnt understand the story, he will hold that as a negative against the game.
As well he should. Spinoff games, movies, novels, etc are meant for side stories that, while they may enhance the plot and understanding of the main series, should not be essential to said overall plot. If I go straight from Two Towers into Return of the King and am suddenly lost plot-wise because I didn't watch two straight-to-home-video side movies and listen to an exclusive audio book and play a video game tie-in released on the Atari Lynx, I would be rightfully pissed off and lose interest.
So as far as I know from playing the main entries in the series, Xehanort / Xemnas as they appear in KH 2 are the true main antagonists of the series. If this particular plot point goes any deeper than this without being first revealed in KH 3 (spoiler alert: I already know it does.), then the series has quite simply failed to properly deliver this aspect of the narrative.
Birth By Sleep and DDD aren't side stories though. BBS is KH0. The're just as important as 1 and 2.
That doesn't refute my point, it actually helps it. They should have been main series entries if they're so important, or they should have had their stories woven in as part of a main series title, like Roxas' intro in KH2 (another format option would have been FF 8's Laguna flashbacks). If they're too complex to fit into a main series game, then the creator / writers need to learn how to edit the fluff.
There's really no excusing shuffling vital overarching plot points into side games to the point where numbered entry-only players will be lost, and if I'm being perfectly honest, I hope KH3 gets critically eviscerated for it.
You have no point when the director of the series considers BBS and DDS to be just as important as the numbered entries and not just side games. ---