The Satyrs
These demonic creatures are the epitome of deceit and trickery. The one thing they hate the most, however, is being tricked themselves. The only way I can see this happening is if when the Burning Legion comes back and Sargeras basically tosses aside the Satyrs and says he has no need for their foul tricks anymore as he begins his full invasion of Azeroth. That would piss the Satyrs off enough to rally against the other demons. This also gives players a demonic playable race, which has been a popular topic for years.
The Satyrs would have to be a neutral race. Malfurion would never allow them to become completely Alliance after what Xavius did and I doubt Thrall would be very ecstatic with them, either. Still, the Satyrs are master infiltrators and that would be extremely handy against the Burning Legion. There will no doubt be loyal Satyrs who remain with the Legion even after Sargeras’ cruel decision. Being able to infiltrate the Legion and bring back intelligence is something both the Alliance and Horde would find valuable.
And, honestly, the only other demonic race I could see being playable without introducing a third faction just for demons would be the Tothrezim since they’re almost just like the Ethereals in their ways of life and greed, just demonic. Satyrs just look like the best choice for a playable demon race that makes sense.