I'm leaning towards Azshara too.
Kul Tiras and Zandalar are still missing. They have been on this trend of filling out the missing areas in the game the past few years. I think it is likely to have an island expansion because of this.
No disrespect, but the mere thought makes me sick. Personally, I feel Kil'jaeden, Azshara and N'zoth could all be final bosses of their own individual expansions, so to see two of them demoted to sharing a mid tier raid is shocking. ( In my mind it would be Kil'jaeden = Argus, Azhara = South Seas, N'zoth = Emerald Dream, Sargeras = Xoroth or some Legion or Titan world)
But to each their own
At this point, I'd say we're more than ready. We fought him before, and we've known he's been out there for.. 6-7 years now? So much anticipation! I'm sure we'd be able to take him on with the help of Velen and Turaylon or Illidan. Which would be awesome to see Illidan working to kill his old boss.
Also after Kil'jaeden I'm sure Sargeras will return to pick up the Legion and come back at us later.
I would need to see what the next expansion is before making a good guess
Kil'jaeden if it's burning legion.
Sargeras we kill in a few expansions later.
N'zoth
I love old gods, they are the most badass thing in the game and lore.
Kil'jaeden would prolly be next on my badass list.
Kil'Jaeden and then that leads into Burning Legion for the xpac after.