For a demon-themed expansion, Legion has surprisingly few demonic bosses.
EN: 0\7 demons.
ToV: 0\3 demons.
NH: 3\10 demons. - here I count Gul'dan,as well - so it's Tichondrius, Krosus and Gul'dan + possessed Illidan (on myth).
3 actual demonic bosses out of 20, first tier.
ToS: at best, 5\9 demons: Infernal pitlord, "Inquisitor" guy, Avatar, Kil'Jaeden and maybe one unknown boss. Others are elf and naga.
So, we have 8\29 demonic bosses in two tiers.
So I do hope that on Argus we will not have any sideway distractions and do what the title of expansion says - kill the demons.
And there are some known from lore who just have to appear.
Namely, Talgath, Hakkar and Mephistroth. Who else do you want to see and fight?
I really want to see the "Grand Inquisitor", to know something more about those mysterious demons.
Also, big Imp Momma would sound fine.
Maybe, a biggest fattest Mo'arg Brute is a good first patchwerk boss for the raid.
And of course, Sargeras, the endboss of the Legion, is the most anticipated guest.