Kingdom Hearts II Axel in the future?

I would think so. Axel could come back.

(It happend in Ar Tonelico. One character died but came back in the second but in a different form)

 

Well, if he hadn't died, then the door to his room in TCTNW wouldn't be red ;P

They are destined to fade into the Darkness, right. I think they will still exist as long as there is still Darkness, I mean, there is Darkness in almost every heart... If Kingdom Hearts is made of Hearts, then it's both Light and Darkness, so the Organization must've gone back to Kingdom Hearts, where they will reunite with their Hearts and be reborn as their original selves. At least that's what I think...