i had, what i thought at the time, was a decent solution. see below.
rather than melding everyone together. offer a third faction composed of elves and displaced groups like worgen and nightbourne, night elves, void elves, blood elves and make it optional.
then horde would be almost back to its original - orcs tauren trolls undead and alliance would be human dwarves kul tirans and gnomes.
then would come the task of making the non elf factions popular again by giving them irresistible allied races. else the third faction would rule the world of warcraft cause elves.
for the horde, to spice it up - san’layn or ogres
for the alliance, to spice it up - high elves or decursed naga