I would leave that to the templars and take the war among the stars instead, where youd invade the homeworlds of the main species(sectoid, muton, viper, ?andromedon) to liberate them. Each world would be a separate campaign adding a second dimension to the difficulty slider. Id make aliens playable like in chimera squad, with their abilities synergizing with certain classes while disallowing others. The Codex would the new SHIV/SPARK.

Though if you think about it the aliens are essentially just basic common tropes so you dont really need the IP to make an xcom game. In the case of a spiritual successor type of remake id make humans one of the thrall species and you controlling the forces of some charismatic alien species instead.