When it comes to the story told in Chi so far, it's really not all that in-depth. It introduces a few new characters/concepts & maybe hints somewhat vaguely at what kind of story threads KHIII will have, but overall so far it's not even nearly as important as other spin-offs like Birth By Sleep, Chain of Memories or Dream Drop Distance and is more in the realm of Coded.
Currently, all that it does is introduce the Foretellers & the Book of Prophecy, sheds a tiny bit more light on the Keyblade War and hints at Xehanort's (or someone somehow assosiated with him) role in all of it. If Foretellers are introduced to KHIII, Sora & Co will need to have them explained, so that's not much of a problem for people who haven't played Chi. If the book of prophecy is important for KHIII's story, that will also need additional exposition in KHIII as to how it relates to KHIII's story and not just its role in the Keyblade War. The Keyblade War stuff itself is nothing deep, just a bit more background info about it. And why a certain seen-in-the-future Keyblade looks like it does and how it ties to the Foretellers is also something that will likely be revealed or much expanded upon in KHIII rather than something that will be thoroughly explained in Chi and left without much explaining in III.
And Chi's whole deal about
will possibly just tie Dream Drop Distance, Chi & III's events more closely together, but unless Chi totally ramps up its storytelling, it's not something that will leave a lot people, who haven't played the game, completely in the dark.
I mean, if this stuff is as important to KHIII as Nomura implies, then most of it will need to be expanded upon & explored with a lot more depth in KHIII itself. Chi might introduce these important characters & other stuff, but if they are to be brought into the "future" somehow, their existence/relevance does need to be explained, and that explanation will have to include some kind of briefing as to what/who they are.
So far Chi is more like how Coded teased the return of MX & the Mark of Mastery that were then more thoroughly handled in DDD, which made Coded almost obsolete from a story POV because all of it was explained a lot better. I see the same happening with Chi. It explains the origins of the Keyblade War and introduces some characters & concepts that might be important for KHIII somehow, but to have those characters/concepts be important in KHIII, their existence/importance in the NOW will have to be explained and that will likely have to include some kind of explanation as to who they were in the past.
Take something like the Book of Prophecy. Ok, Chi players already know it exists and will have a rough idea what it is, but if they are to introduce it as some kind of important piece of KHIII's story, they'll need to justify it somehow and that justification will likely include some kind of explanation as to what the Book of Prophecy even is. So people won't be left in the dark too much vs. Chi players. Sora won't just suddenly start talking about the Book of Prophecy in KHIII as if we should know what it is. Some villain might mention it first before it is explained and that's when Chi players will have a slight advantage in knowing roughly what they are talking about, but there will surely be a point in the game where Sora & Co will get an explanation as to what the BoP is (if it's important to the story).
