I was sure this is how they were gonna go when WoW was first announced. Frozen Throne set it up perfectly to have Horde, Alliance and Illidari faction with naga, belves, broken and demon hunters.
Never was sure why they decided to go full villain with Kael, Vash'j and Illidan in TBC. They were the most interesting characters in TFT imo.
Maybe 3 factions would be a little to much but I don't know, ESO for example is doing fine with it.