In one of the first releases there was an alien campaign. It was rather silly- all the same as in normal, just the soldiers had alien appearance. Was it just an early concept that was abandoned?
In old Xcom games I always wanted to play as aliens, destroying human cities and searching for Xcom bases
Actually it would be quite nice idea to make an alien campaign after the human one is complete-like the aliens have lost war against humanity, human invaders have launched a massive invasion, and their forces are pillaging the alien world. Desperate aliens chose a new commander who's task is to first eliminate human from alien world. And than, go on a suicide mission for a final assault on Earth to make sure humans will not have the chance to send another strike. Suicide, because ships would have just enough antimater for a one way trip on earth, and after arrival the mission would be to destroy human AM storages/more advanced ships, before overwhelmed and destroyed. Such an end would leave the game open for future development, as none of the forces would be eliminated.
Humans would have high level technology- lasers, plasma and so on, but as in the campaign they would not be able to produce particle weapon ammo, so its usage would be rare and connected to lost missions. Aliens would be able to develop new technologies, while humans would be stronger and better trained (years of battle have done the job)
Of course its just an idea, as the main campaign is far from being fully implemented.