Elites of course. They are a gimmie, or at least should be in my mind. They have a great following in the Halo franchise, and it's just cool to break out of that norm of playing as a Spartan. Not to mention Elites are some of the most honorable aliens I've ever had a fight with... except when they Karate Kick me on Legendary, that's where I draw the line.
For other aliens however, there could be a mode somewhere for this, maybe a little like Beast mode from GOW3, but just to the extent of "You play as all of the enemy species." Now playing as a Promethean Knight or Hunter... that would be a scary thought that another person is controlling him and is on the move looking to get you.