I kinda feel like Death Knights are the undead-adjacent thing, only instead of being a race, they’re a class.
I’ve two suggestions;
Vargul, the undead vyrkul. Lore says they were always sapient and enslaved the old fashion way by Arthas so Lord knows why they didn’t have a Taunka esque subplot for the Forsaken in Wrath if nothing else.
Having big draugr friends would be cool. Have them ride around on plagued proto drakes. Constantly challenging Tauren to arm wrestling matches.
Or, instead of the bigs, we get the smalls, with Leper Gnomes. More than a few leper Gnomes already work alongside RAS so it would feel pretty natural. Having the scamps running around being mad science bros with the Apothecaries sounds fun.
The Drakkari spirits in The Other Side wanting to return the Zul’Drak and be an Undead Troll Civilization that works with the Forsaken and Zandalar would be an amazing concept