Nexus: Qhira - has no lore. Doesn’t count.
Warcraft : Iirc Lunara, Brightwing and Murky were first introduced in HotS. Not sure if Murky was first introduced in the book “The Traveler” though. However all three have been canonized in Legion, however Brightwing and Murky got their own quest chains, while Lunara is just a random Dryad chilling around with the Druids doing nothing, so she’s a candidate. There’s also E.T.C that’s been there since Warcraft 3, but he never really fit anywhere. So Lunara and E.T.C qualify. However there’s also Anduin, his lore sucks really, but he has lots of it, so doesn’t count.
Starcraft : Iirc Blaze, Lt.Morales, Sgt. Hammer and Probius were first introduced in the Nexus, and to my knowledge all have appeared in SC2, but with small cameos, and almost no lore at all.
Diablo : All heroes. Their names and even genders don’t even have to appear in game. As some people said on my topic(Diablo heroes in HotS) it’s been hinted in Diablo that those are canon names and genders(at least for some) and some will appear in Diablo Immortal, and have their short stories, but still.
Overwatch : All heroes in hots have their Pixar quality animated shorts(Tracer even has quite a few) except for Ana, Lucio and Zarya. Ana has a minute long Origin, while the other two have none, so they qualify.
TL;DR (man do I write walls of text) imo it’s E.T.C simply because there are dozens of Tauren possibilities, and they added a Tauren that doesn’t fit anywhere and looks just weird.