Shocking cliffhanger. Such a tight open ending you pretty much understand why they had no idea where to go after it.
I don't really think it was Kamui who got stabbed since he's clenching to his own Shinken when Fuuma has him cornered, that has to mean something and it could have went both ways, which is why I think it's such a great open ending.
If anything, I theorize Fuuma has a death wish, a recurrent theme on the later volumes, that only Kamui can grant him; and has delayed killing Kamui because of it. Meanwhile Kamui's "true wish" may have had to do with how he truly feels about the destiny of the Earth: he aligned with the Dragons of Heaven only out of a sense of duty to Fuuma to get him back to normal, but admitted he may have been making a mistake since the future of the Dragons of Earth was far more laudable to him on the long term. There's also the fact that any of the Dragons of Heaven who had lost their ability to create Kekkai is destined to replace a fallen Dragon of Earth, and I think that's the path the story was going to concerning Kamui's inability to create Kekkai: Fuuma, or the "evil" Kamui, is destined to die, and "our" Kamui to replace him as a Dragon of Earth and create the future Kakkyou predicted.
But we'll never know. Damn CLAMP.