This is just me tossing some ideas out there for discussion... Not a yea/nay or anything like that, because I'm rather interested in this myself!

Neltharion wasn't always the dragon Aspect of Death. He used to be the Earth Warder, the head honcho of all things related to the earth of Azeroth. It wasn't until he had to suffer the trauma of the Sundering compounded with sanity-nibbling whispers from Old Gods did he become Deathwing. That being said, we have some options:

1) The Redemption Ending: We had the same speculation (dare I say, hopes, too?) for Arthas at the end of Wrath. The ending to the questline in Twilight Highlands involving the red dragons seems like it might be a no-brainer for Alexstrasza, the red dragonflight and the heroes of Azeroth. Deathwing is completely bat-shite crazy and needs to be put down. But Neltharion and Alexstrasza were as best friends as major dragon aspects could be! Even if she says he must be put down for once and for all, she doesn't really mean it... does she? Maybe deep down inside that molten body, he's still Neltharion?

2) Kill him: As stated before, Alexstrasza says to Deathwing that she must end him. But how is that going to affect Azeroth itself? We killed Malygos in Wrath because he was going nuts and it didn't seem to do to much to the world as we know it, perhaps because Malygos wasn't tied to anything physical of the world (in the literal sense). But Deathwing is still the Earth Warder, no matter how twisted and tormented he is. Ending him might result in something worse than the Shattering, or something getting loose from the earth and Deathwing was only a distraction and/or beginning.

3) Another will take his place: The questline in the Badlands hints heavily at something (at least, in my opinion, it does). If you haven't done that chain yet, it's very very interesting-- That's all I'm going to say to avoid spoilers. *IF* this questline is as significant as it seems like it is, then we might have a combination of the first two options as the ending to the expansion... We kill him but there will be someone new already set up to take over the reins to completely redeem the Black Dragonflight (whereas the Blue Dragonflight is still leaderless at the moment). The only caveat to this idea is that the Black Dragonflight may not accept the new leader at all, which results in having to create an entirely new Black Dragonflight and eradicating all of the old.

Anyway... Like I said, this is all speculation and just me thinking out loud. I honestly hope that there's more in store for Deathwing/Neltharion than just "Kill him and loot him" at the end.