The final boss in Legion?

[Lime]

Probably Velen corrupted.

I'm still a firm believer that he's trying to pull a Darth Sidious.

[Benomatic]

He's been playing both sides the whole time?, setting up all the pieces, bringing the exodar to azeroth, allowing the legion to finally get hold of the remainder of the draenei refugees?being ranked above Kil'jaeden and Archimonde the whole time?

[MatthewOU2015]

Hopefully it's a female. Tired of all the male final bosses.

[baatora]

The Avatar of Sargeras: Yrel.

[dwightyo39]

It will be a blood dk or prot paladin.

[Necroofeelya]

It'll probably be Archimonde again

It probably will not be Archimonde. We already killed him. I know technically Demons do not die, they are just banished to the twisting nether. Kil'Jaden is more than likely though. The most we have done to him is stuff him back down the sunwell on his attempt to emerge into Azeroth.

the replacement of Archimonde.

I can see Velen becoming corrupted and taking his place. Other than him there is no one really. Kil'Jaden and Archimonde are eredar from the original home world of the draenei. They were recruited by sargares himself and so far it seems like the only "legion" lords have been the main leaders of draenei on the home planet that gave into the power of the legion.
Which leaves Velen as the last candidate.

Velen, Kil'Jaden, and a Avatar of Sargares all have an even shot of being the endgame boss and I am excited if either one of them make it.

[Golden Yak]

Kil'jaeden's not a bad possibility, actually.

Illidan's plot in BC was recently revealed to be the destruction of Argus and the death of Sargeras' strongest lieutenants. Would be a worthy way to finish Legion - not an outright deathblow to the Legion, but dealing them a serious setback and stripping Sargeras of some of his mightiest servants.

[Aeluron Lightsong]

Avatar of Sargeras/ Shadow of Sargeras/ Echo of Sargeras would be a great end boss.

[In Ogres We Trust]

The Avatar of Sargeras through the body of Illidan.

[Gen4Glock21]

The demonic versions of Voljin and Varian.

[Twistedelmo]

Undead Garrosh

[AwkwardSquirtle]

The Avatar of Sargeras: Yrel.

Not quite how the avatar worked. The avatar of Sargeras was just a partial manifestation of Sargeras. It was capable of possessing people, but to do so he had to inhabit Aegwynn's body and then possess Medivh, and even then I don't remember Medivh ever being referred to himself as the avatar.

Also they're probably going to pretend WoD never actually happened, so no Yrel.