As long as we're just throwing junk at the wall here, why not shoot for the moon? How awesome would it be to turn the tables and play as the nefarious aliens trying to [spoiler redacted]? Instead of a full-blown triple-A experience here, which would almost certainly exceed the scope of DLC, an asynchronous multiplayer mode would be a great fit.
Imagine being able to log into your game and see an alien's eye view of someone else's existing single-player campaign. From there, you could schedule abduction, terror, and other missions to try and fulfill your objectives. Maybe successfully abducting a certain number of humans would accelerate the deployment of advanced alien types, or panicking certain countries let you set up additional defenses and traps for the XCOM squad to run into while they're out trying to thwart your master plan. Maybe you could invest your spoils in alien heroes that are even nastier than your typical muton. There's no reason you couldn't tap into this kind of asynchronous multiplayer experience from a browser or smartphone, either.
On the other side of things, victory would be that much sweeter by overcoming an intelligent adversary rather than a series of preset algorithms.