At this point it’s going to be nearly impossible to fix the faction imbalance. Even if you buffed alliance racials to be better than horde ones or you give free faction changes, it’s not going to do much since the raiding population has been moving over to horde since cata.
It’s really just a player problem now that can’t be fixed and really the only thing that will fix this problem so late in the game’s life is getting rid of the factions.