Because people don't think something can be beautifully tragic without a character dying I say. I don't think anyone is going to die in KH3, every character has somewhere they can go in the end. Except the bad guys, they'll all most likely die.
Sora - MC through and through, Nomura said KH is his story so he'll never die.
Donald, Goofy, and Mickey - Disney would never let their characters die and they'll return to Disney castle/world/land/town
Riku - MC by default, if it wasn't Sora then Riku would be next in line. He's the KH equivalent of Mickey at this point plus he's a master.
Kairi - Pure Heart, Princess of Heart, and now keyblade wielder plus the MC love interest. She's been waiting all this time to be with Sora there's no way they'd have her die.
Lea - He's human, he has friends to save and he's died a few times in the series now his death would have no real impact. Definitely may see some tragedy between him and Isa when he's taken down.
Roxas - Part of Sora's goal is to bring Roxas to life so even if he comes back on the wrong side I seriously doubt they'd just kill him off.
Xion - she looks like she may be a part of the darknesses, and there's no telling what her face will look like, but being just a shadow of Sora I can't see that she would survive, especially on the bad side. Although, if Roxas has anything to say about it I'm sure that he would be able to save her.
Namine - It doesn't look like she's going to be involved at all, but because she's on the cover art I'm gonna throw my hand in and say that some book with data inside is going to allow her to be brought to light with the help of Kairi.
Terra - I doubt they'd go through all of this to bring him together just to have him disappear again. That would be absurd.
Aqua - following Terra's train, after everything the players have gone through to save her from the RoD and the fact that Sora fights her (maybe) I doubt they would just be like aaaaand she's gone.
Ventus - same thing as above, except he may disappear back to his original time if the worlds aren't unified to UX status at the end (which I think they might).
Riku Replica - he'd be the only plausible death because he was just a copy, one of Vexen's clones, and might already be on a shorter life span due to clone status. However, if Namine comes back she wouldn't really have a place. Roxas would have Xion and Lea (maybe Isa) and Namine could have Riku Replica because of his promise to her.
Data Riku - he's on the bad side and he's made of data so I don't see him being a survivor, especially not with Replica because then there would be three Riku.
Vanitas - he's the embodiment of darkness and has no reason to try and be redeemed, there's a good chance that all of what makes him Vanitas will be given back to those he got it from (Ventus and maybe Sora based on appearance).
Xehanort - could survive, especially since his plans will probably come to fruition depending on why exactly he was trying to recreate the old world. However this is the end of his saga so there's a good chance he disappears after it all ends.
Braig/Xigbar - most definitely will disappear or become whole unless he's actually pulling strings like many are lead to theorize.
Even/Vexen - he can go back to Radiant Garden with the rest of the crew and continue his scientific research.
Demyx and Luxord - most likely will disappear, other than the manga they haven't had much of any sort of redeemable qualities and we don't know anything else about them. We don't even know where they are from. Hopefully we'll learn more in KH3?
Erlena/Larxene and Lauriam/Marluxia - same as Ventus, they all have a connection so if the world is restored to the way it was then we could see them returned to normal and continuing his search for his sister.
Ansem, Xemnas, and Terranort - I've always preached that I want to see Ansem and Xemnas or a combination of both be consumed by Lingering Will similar to Armored Ventus Nightmare and have the shell break to reveal a newly revived Terra, and what we saw in the trailer may confirm just that. They are all copies and time displacements that won't exist at the end as they are one of the same entity.
Young Xehanort - Being a time clone I doubt he'll do anything other than fade back to his own time as well.
Isa/Saix - This is the only one I'm unsure about. He could go either way. In 358/2 he was aware of his old relationship with Axel but it didn't seem like he cared so much as he was using it as an excuse to manipulate Axel into doing his job. In KH2 he didn't seem to care about Axel at all and he mindlessly attacked Lea without any hesitation so I don't know if he's too far gone or if he might return as a somebody. Also, Lea seems to have moved on and is more focused on Roxas, although when he sees Xion he may be compelled to try and save all of his friends, Isa included, side by side with Roxas (who I feel will be a good guy).
That should about sum up all the possible lights/darknesses so if anyone else wants to chime in feel free.
