This.
In general, I don't see a reason not to just merge the two games together. It really feels like they belong merged; TFTD always felt like an expansion to me.
For instance, build a base: on land? You get an EU base; in water? You get a TFTD base. UFOs cannot be intercepted by USOs and vice versa, but the type of mission (and thus which craft can land for recovery) depends on where they crash. Solders are soldiers everywhere (marines!), and can use all the weapon types. The weapons are even diverse enough as it is, Gauss weaponry is really quite unlike Laser weaponry and there are uses for both (and you might want to bring Gauss weapons for attacking surface enemy bases, and Laser weapons when attacking boats, for instance). Both research trees would be available at once; there would be both alien alloys and aquaplastics, Elerium and Zrbite, used for the different kinds of technology. And I'd imagine that in order to win, you'd first have to destroy T'Leth, and only then Cydonia, since the latter is quite a ways off.
It's a huge undertaking for sure, and would need altering a few texts to make it cohesive, but I feel like in the end it's worth having one, single, united game with all features, instead of two separate games on the exact same engine.
