They had a FFA PvP server. Or even a racial PvP server. Not sure how this would be a downside as people who dont like FFA can do their thing still, and renegades such as myself can mess around on our own server. I know WoW lore doesnt support this but if you still think Blizz cares about that then you are sadly mistaken. Look at EQ and DaoC, they both had FFA servers in addition to normal game types, and it worked fine.
If they brought this in MoP Id resub instantly. Always liked WoW gameplay, just not the character advancement methods they currently have (do DF runs until 85, then grind for PvE or PvP gear)