Honestly, what I think is proven time and time again is that factions don’t work modern-day unless strictly enforced and even then it only works when you have a small number of realms for players to play on.
Arguably the game would be much better off without them, and allowing those players who have that kind of desire to stay on their side without being forced to.
A guild who only accept horde/alliance races, a group of people who go out wpvping horde/alliance races specifically (hell maybe even attack your own races for the spice of being a bandit or something).