Firaxis have an obscure tease of a potential sequel to XCOM: Enemy Unknown (the new one). Admittedly it is pretty light on hype and big on speculation at the moment. Some people have mentioned the original sequel to the original game, which I bought and played, featuring an underwater threat.

The only thing I remember about XCOM: Terror From the Deep was that it was much harder than the original, and the aquatic aliens got psi-powers real quick, often killing my guys and making me reload in a fit of frustration. Hopefully, Firaxis won’t be repeating that particular “feature”, although the underwater deal was at least unique.