I would say probably Necromancer or Lich class, if we were going to get any new classes, these two would be one of the options I'd want.
Necromancer Races:
Alliance:
- Human
- Night Elf (Because they don't have access to Warlock, so maybe they'd raise the fallen elves?)
- Gnome
- Velf
- Dwarf
Horde:
- Troll
- Orc
- Undead
- Goblin
- Nightborne
Lich:
I'm not really sure what races would work well with a Lich class, but I'd 100% imagine that maybe Undead would work well?