I believe that maybe due to time all the diablo characters where cut short being that it would have been great taking advantage of them being able to be the only heroes in the game that would have a gender option being able to chooser between a male or female witch doctor, barbarian, etc… (of course this implies to the player created characters from Diablo and not the diablo bosses or angels.) PS its also a bit of a pain in the neck every time i see nazeebo or kahrazeem or valla, none of these names are even famous lore characters names in the Diablo series, i don’t ever remember making a Sonya but i do remember making a barbarian and choosing male or female
i already have a player name for everyone to see i dont appreciate u making one up for my Diablo Wizard
They were made into the lore character representing that type in Diablo III. Similar to how Diablo III named various characters from Diablo II such as the Sorceress (seen as a spirit in act 5 when playing Wizard).
Male Barbarian did exist at some stage as indicated by old data leaks. I suspect that at some stage they were intending for both male and female versions of the characters to exist as separate heroes with separate skills (representing different builds) but sadly that never happened.
This.
What’s more, they also created names for the D1 characters for D3:
Spoilers, I guess
In D2, the warrior who became the Dark Wanderer wasn’t named, but he turned out to be Prince Aidan in D3.
In D2, Kashya said Blood Raven was “one of our finest captains in the battle against Diablo at Tristram”, but the dialogue seemed to indicate that she was called Blood Raven even then; later, it was retconed that she was called Moreina, and only “became” Blood Raven when she was corrupted.
Similarly, “the Summoner” wasn’t named in D2, and the name Jazreth was only given to him in material relating to D3
And it’s actually the same for Starctaft:
The Terran Commander and Protoss Executor that led the armies in SC1 and BW were “blank slate” type characters, but SC2 retconed them as Horner and Artanis, respectively (which never made sense to me, since Artanis interacts with the nameless Executor during BW).
I don’t think the Zerg Cerebrate is ever even mentioned.
My point is, “replacing” the in-game player characters with specific ones for the lore is how Blizz deals with that, and has been for a while now.
Also, I agree with what you say here about Naz and Khara
but Valla does have gear named after her, and she’s one of the only two who were taken directly from the short stories in the D3 site that introduce the classes (the other was Li Ming), so I think she was canon even before D3 actually launched 
.
On the other subject, if the game was still in active development and getting more than a handful of new heroes each year, I would have liked to see the other-gender versions of the D3 playables, with a different skillset.
(as DrSuperGood mentioned, there were datamined assets for the male barbarian, and also the female witch doctor IIRC, I believe their work-in-progress names were Khronan and Nazeeba, neither of which I like, but that’s besides the point).
Or even just as skins, like they did with D3 Diablo
But as it stands, I’d rather get different characters instead of “doubling up”.
Give us Baal, Andariel, Duriel, Izual, Itherael, Adira, or some other big character who hasn’t made it in yet, instead of a class-representative for an already represented class.
Of course, that’s just my opinion.
It is my understanding that Artanis only became his StarCraft II position after the events of Broodwars as the various Protoss factions started to unite.
In line with what you said though, he was only given a unique character design with Legacy of the Void. Back in Wings of Liberty he used a generic executer portrait for his mothership.
They were all meant to have been destroyed by Kerrigan at the end of Broodwars so she could assume absolute control.
What changed was in StarCraft II the Cerebrates, and even the Overmind were made into Amon’s puppets.