Actually, I'm going to go ahead and do something I rarely do. Disagree with skyfoxx. Though there is a point I don't understand about it, I believe Kingdom Hearts MAY be in the Realm of Darkness. Hell, we DID see it in KH1. If you look behind the DtD, you can clearly see a (black) outline of a giant heart. Kingdom Hearts. Opening the Door to Darkness is the natural way to lead you to Kingdom Hearts without all that mucking about with trying to create something like the X-Blade (sure, you have to go through a bit more trouble of collecting all 7 purest hearts, but...whatever...) The one thing that doesn't make sense about Kingdom Hearts being in the Realm of Darkness is this....the Heartless's nature is to seek out hearts. It has been stated many many times that Heartless "want to return to the greatest of all hearts." Derp, that's Kingdom Hearts. Well, if Kingdom Hearts is in their natural Realm, then...uhh...why don't they just...go to it...? Like, why bother wanting to come to the Realm of Light for hearts if you have the greatest of all hearts sitting in your backyard? It just doesn't make too much sense.
And also, I am wondering where indeed you got that Kingdom hearts is both Light and Darkness. In the Tale, the "Light" that everyone began to fight over is almost confirmed to be Kingdom Hearts. Kingdom Hearts is the purest of all light, seeing as it is the hearts of all worlds...in the realm of Light...Nowhere that I am aware of was it said that Kingdom Hearts is both. The only thing we know about it is that if one were to pass into it, they would gain phenomenal power and wisdom, and ascend to something beyond human, and into a completely different "realm" separate from anything we know of so far (and the different ways to unlock it...)
Also on a subject that was brought up earlier.....When Ansem SoD called out to "Kingdom Hearts" and the Door opened, darkness began to pour out. Why? Because the DtD leads to the Realm of Darkness (herp derp), so obviously Darkness would begin to pour out. But then, after what Sora said, deep down Kingdom Hearts reacted, and bursted through the darkness with its Light. THAT was the Light, not Mickey. That's just...stupid. He hadn't even arrived at that point. A few seconds later, yes, but...not yet. And even so, why would he burst out with Light, go and hide somewhere, wait for more giant Heartless to show up, and then fight them off...again...?
So...yeah.
BUT ANYWAYS THIS IS ABOUT THE NEXT PART OF THE SERIES.
So here's what I think....I have no idea.