Blizzard have announced a new feature; Mercenary Mode. Mercenary Mode is a feature that will allow characters to fight for the opposite faction as a mercenary. As a mercenary, you will be seamlessly integrated into the enemies team as one of their own. Mercenary Mode will only for Ashran and unrated Battlegrounds.
In addition, when you join the other faction's team as a mercenary, you will become that race entirely. This means you will gain their racials, passives and of course the look of that race. You have the option to not be a mercenary of course, but if you want to become a Mercenary, you can talk to an NPC either in Stormshield or Warspear to enable Mercenary Mode.
Concerns have been "raised" about players who chose the Human race, as Humans get a racial that allows them to remove Crowd Control effects, so they are looking into other options besides Humans having to spend Conquest or Honor on a trinket to replace the racial's effects.
If you ask me, they should simply do racial abilities as they do talents; let people select a single racial ability, a single racial passive and make it easy to change using Tomes of the Clear mind. This would then remove race imbalances and instead mean that your selection of race is purely cosmetic.
As for the actual Mercenary Mode, players have reacted quite negatively, with the only real complaint being that players from other factions will try to sabotage a victory when placed on the opposite team. This is a very real concern, as people really do have hatred for the other faction and I'm sure there are some players that would stop at nothing to waste everyone else's time.
As for me, I think its a very cool idea. I like the idea of being able to play as an Alliance character without having to faction change. The reason why Blizzard have introduced Mercenary Mode is because the queue times for Alliance are much longer than Horde; for both Ashran and unrated Battlegrounds. The sheer number of PvP players on Alliance heavily outweighs the Horde's numbers, I'd blame this purely on the fact that all the competitive PvPers have gone to Alliance so that they can play as a Human, so that they can have another damage trinket. Indeed, racial imbalance is currently a huge problem in PvP, and needs to be sorted out.