Since the scourge started in Lordaeron, most of its victims (and recruits) would’ve been human, so the Forsaken are predominately human. The only other playable race that I can think of that makes sense is undead high elves since the scourge later made their way to Quel’thalas, but they just recently added undead elf customizations.
As for any other playable races, their numbers in the forsaken are probably seen as too small to justify making them playable body types and Blizz might be afraid of handing over other race’s identities to the Forsaken (hence giving Darkfallen customizations specifically to the Belves rather than to the Forsaken themselves).
With the dark ranger skin, can be technically an undead elf of 3 different types
(edit: There was a probably fake leak some time ago of humans getting a nathonos style skin, but dunno how real that was, was just some random youtube video)
Technically possible, Sylvanas is restricted to low tier Valk’yre/soul healers, none of which are high ranked Kyrian/Valarjar…
Bolvar inherited from the prior Lich King: a cult of Necromancers, the Deathlord and the Twin blades of the Fallen Prince (daggers forged from Frostmourne), and a few high ranked Liches of reputable study in necromancy.
Most Worgen we see outside of the starting area and hubs like SW are feral, they haven’t undergone the ritual to gain their old sane humanity back, supposedly the only reason DK Worgen have their clarity of mind back is because the Helm of Domination “broke” the beasts within (the fact that this is standard for Worgen DKs as an additional step suggests that most if not all of the Worgen corpses raised as DKs were feral).
mind serum from the starting zone in Gilneas+ Druid ritual later in the Worgen starting zone=immunity.
I don’t know that Blizzard would add extra mangled and showing bones models for warcraft since this game’s primary target market is China where showing bones apparently scares their censors or something for being too spooky.
I’m sure they could make it work but keep in mind I was thinking of the limitations they were working with in the past, prior the cross-faction options.
And even then cross-faction has its limitations. Why? I do not know. I hate that I can’t quest with my non-Horde buddy in the open world.