everything would feel off if the next warchief wasnt a well known orc or troll. taurens, blood elfs, and goblins(not all) certainly will never have much pull in wow, im sorry but they are secondary at this point. while undead are pretty big, sylvanas hasnt done much in recent times, so it would be kinda weird for her to become warchief "hey undead lady, if uh you arent doing anything right now do you mind becoming the warcheif of the horde for a few mmmkay? also considering the undead dont really care much for the horde(established from wrathgate) its not not going to happen