Hm. I know this prolly won't be true, but what about a persons light? It isn't, or can't be snuffed out, as we see with Terra. So what if we have heartless counter measure. With all the new enemies, and a possibilty that Unversed would return, it seems over whelming, so the light manifests, becomes something like a heartless, but it keeps them at bay. Just seems insane to add another thousand of a 4th enemy group
I was thinking that perhaps they could make enemies out of damaged somebodies: basically, these would be the nobodies and hearts that rejoined with each other, but instead of coming back as ther original somebody, they are twisted and imcomplete.
Ok, I insanely love both of these theories, and I think they work really well together.
When darkness takes over a person's heart and they become a Heartless, maybe the darkness expels the light within? As Sora said, the heart may be weak, and sometimes it may even give in, but deep down there's a light that never goes out. Darkness can never fully engulf light, but maybe the heart turning into a Heartless releases the light to form a light-based counterpart to try to stop the darkness, a defence mechanism as aldrain said.
Well, we actually haven't seen any of these light counterparts yet in the series when someone becomes a Heartless, meaning they would have to have been sent to another world when formed, like Nobodies, probably to somewhere in the Realm of Light. Not a fantastic defensive measure if it catapults them across the galaxy, is it? Maybe it's just that the darkness shrouding the heart sends the light packing to the RoL, subsequently creating a new light-based creature. Which brings me to Absolute's theory...
When a Heartless and Nobody fuse together, maybe they need the light being as well to become a complete Somebody once more. Without the light balancing them out, this new entity would be far more advanced than just a lone Heartless or Nobody, but still not quite a Somebody. They could have a heart and darkness from the Heartless and a mind, will, memories, body and soul from the Nobody but without this third creature of light, they are not complete. These could very well be a new enemy. Organization XIII went to great lengths to try to possess hearts once more. Maybe these new entities will be the same, except with light.
I know that, going by this theory, Sora should be the same kind of creature after merging with his Nobody Roxas. However, Kairi used her own light to save Sora from his Heartless-ness, meaning he did have light. Also, Roxas, having Ven's heart, wielded the element of light in the Organization. Sora's Nobody and heart both having light when they reformed would have saved him from becoming this other entity lacking light.
Only thing is, this theory of light creatures and junk only works if the light is removed from the heart upon becoming a Heartless. Isn't it a common theme in KH that in every darkness there is a light? Is it confirmed, then, that the light stays at the core of the heart even after it gives in to darkness and becomes a Heartless? Hmm...