Agreed. Seems like it would be easy to do as well. Have a generic DK called the Deathlord or perhaps one of the Four Horsemen raise us up, then do whatever DK stuff.
DH IMO should have something similar. Have a generic DH or perhaps someone like Altruis give characters a way to become DHs, using perhaps something like the story of the DHs within the novel Illidan. Would be really interesting to see IMO.
Setting it on characters that are heavily involved in the storyline like Arthas, Illidan, or Bolvar (seemingly for Shadowlands) only leads to issues where making that class is near impossible on new races once that character goes through whatever story moments.