Warcraft exists for 25 years. There are 3 big strategy games, the best and biggest MMORPG game ever made with 8 huge expansions, more than 30 books, huge amount of comics, separate card game, a movie…
Both, Starcraft and Diablo exist for mor than 20 years, OW has its fifth birthday (it was revealed at Blizzcon 2014). Hundreds if not thousands characters were created for those franchises in years. You can’t add every single one to Hots even if you work at the game for more than one century!
You don’t have to add only the most important iconic characters. You can add minor characters. You can add other NPCs you can find in the game, you don’t even interact with them, they are just there (like Lunara, she is a dryad), playable classes (we have every Wow and diablo class in the game and multiple SC units) and others.
It is impossible to create OC hero, which kit wouldn’t fit to someone from other Blizzard games. Like Qhira and Harrison Jones. Harrison Jones is absolutely a minor character, he is far from being somehow important for the main lore, but everyone know what Harrison Jones means.
Additionally, when you add super iconic character to Hots, he doesn’t have to have exactly the same kit as in the original game. It can be changed to work better in MOBA game or it can be just inspired (Ragnaros).
Do you think, when LoL od Dota 2 devs want to add a new character they are locked into dark room and their boss let them out only after they design brand new kit which never existed before?? Of course not!
When you are game developer you always try to find inspirations everywhere and you really like to “borrow” ideas from other, even competing games. Plenty of LoL champs are inspired by Warcraft or Starcraft characters. Hots heroes are inspired by LoL champs and even newer LoL champs are inspired by some ideas from Hots (Yummi, the magical cat is more than inspired by Abathur). There are characters in LoL, Hots and Dota2 which are pretty much identical. They are named differently and have slightly different animations and colors, but the basic idea behind their kits is the same.
Do you think they have time, money and free people to create some lore for Qhira? Even if they do something like that I highly doubt it would be something else than short comix. Let’s be honest, it doesn’t sound very interesting.
Do you know what easter egg means? Blizzards actually adds characters from one game to another as easter eggs. For example, there is a blonde blood elf in stealth named Nova Terra somewhere in Outland.
But is is extremely silly to think Qhira could be new OW character or Wow boss.