I was watching a treailer a while ago a its a bit old so srry, and you could here a voice (ansem I think) speaking to riku in the heartless world (kingdom Hearts) and he was saying it takes to hearts to seal the door so if you think this may be the end of the series (dont think anyone thought there was a 3) and that 2 peaple will have to die if you under stand what he said. So most lickly the 2 to die would be Sora and Riku. But also if you think of it that would be an odd ending couse than sora Riku and kiari would never be reunited for long. Just somthing to think bout.