Baal from Diablo franchise.
He was the main antagonist in D2L which once led Blizzard to one of the greatest gaming company in history.
And he still hasn’t invited yet to NEXUS?? REALLY?? WHY
I see the game has pretty much ended for creating new heroes, and all the developers has moved to different department. but come on, we all got our main antagonist from warcraft & starcraft franchises, but diablo hasn’t ended still since Baal the prime evil and main antagonist in D2 expansion is not invited to nexus 
Blizzard didn’t make Diablo 2.
Nice try though.
While I agree that Baal needs to make it into the Nexus, I have to say this:
Is quite simply not true.
The main antagonists from SC1 were Mengsk and the Overmind, non of which are in HotS yet (granted, Kerrigan was the one for the expansion).
For SC2 it was Mengsk again, as well as Amon and his Hybrid. Again, not in the Nexus.
As for Warcraft, I don’t play WoW, so I can’t say who’s the “main” antagonist(s), but I’m pretty sure Eredar, Naga, and Old Gods related critters are big ones, and not in HotS.
As a side note, the antagonists for Overwatch are mostly Talon agents (and Omnics in the backstory), so again, none of them are added to HotS yet; and I’m pretty sure the Classics and Nexus hero categories also have only “good guys”.
In short, all the franchises are still missing some pretty big names when it comes to the “villains” of the source games.
But it would still be pretty nice to see Baal added.