Five of the bosses have been identified.
Of these five, three are named. The Tarragrue, Sylvanas and Kel’Thuzad.
Two are known but we know little about them. The Eye of the Jailer is almost certainly the removed and repurposed eye that Odyn sacrificed for his glimpse into the Shadowlands. Is it an autonomous entity now or is someone wielding it?
Similarly the ‘Fate of the Damned’ almost certainly refers to the mass of souls that have flowed into the Maw with the attack on the Arbiter. But who are we killing to liberate it?
Helya is logically a boss we will face in the Sanctum. SoD seems to be the end of the storyline for the Maw and this is where she is now based. She could be involved in either of the Eye of the Jailer (given her connection to Odyn) or Fate of the Damned fights (Helya seems to have a role overseeing the River of Souls) or she will be a boss in her own right.
As for the remainder, we need to consider certain factors.
Players need to be able to recognise the boss. A pre-defined relationship from having encountered them in the past will be best. This means that while Sorceror Thane Thaurissan may have been an evil monster who belongs in the Sanctum, the fact most players have no idea who he is pretty much rules him out.
Demons and void creatures are out, when they die they go to their respective realms. No Archimonde, no Mannoroth, no Il’gynoth.
Thirdly, the bad guys need to be truly the worst of the worst. Garrosh was a bad guy and he got sent to Revendreth, not the Maw. These souls wouldn’t have been afforded the chance for redemption if they were so bad they ended up in the Jailer’s personal prison.
Finally, no Arthas. Too big a character to play second fiddle within Torghast. Now he MAY have a role within the Sanctum (in fact I fully expect him to be encountered) but he won’t be a boss.
The characters I believe we could encounter are
Gul’dan. The man was an absolute monster in both his incarnations. Completely irredeemable, he would have been sent to the maw immediately on both occasions. Steve Danuser indicated that alternate versions of characters from the Prime Timeline share souls, so alternate Gul’Dan likely merged with Prime Gul’Dan upon his passing. It would be fun if the merging went wrong and his soul kept splitting in two during the fight and remerging.
Deathwing. Dragon aspects, when they die, go to the Shadowlands and while Deathwing’s body was unmade his soul likely went straight to the Maw. Blizzard love Dragon fights and this would be an opportunity to right the wrong of the Cataclysm final fight.
Lei Shen. The Thunder King built an empire on the backs of tens of thousands of slaves, ripped the heart from a keeper and brought misery to the lands of Pandaria. Definitely possible.
Argus the Unmaker: If the theory that the death of Argus was responsible for shutting the Arbiter down is true, then his soul needed to go somewhere and that somewhere is the Maw. This would serve to reveal the grand strategy we were pawns in.
Nathanos Blightcaller: He swore to be at his lady’s side, and being a world boss in a pre-patch event and wanting to die to return to Sylvanas as rapidly as possible mean we haven’t seen the last of him. Either a boss in his own right or part of the Sylvanas encounter.
So those are my guesses.