Scorch, you may have misunderstood me, but I didn't say that taking control should be removed. I mean, when you use this feature, it's addictive. You start taking control of your enemies, and you think, " wow, this is awesome!" After that, you can't give it up, because it's so convenient.
What about the tanks. You say that additional weapons must have infinite ammo. But in Piratez, the machine gun runs out of ammo. I'm not a programmer, so I don't know how to do this, but maybe Dioxine can help?
Also, about the psi-weapons. If the aliens don't do this, then maybe XCOM will develop it itself? After all, someone made The Lotus glove. XCOM could disassemble such devices, understand how they work, and then create its own version of psi weapons. After the player has disassembled and studied the available psi weapons(gloves, staffs), they will be able to discover the technology for building their models. Of course, this requires the best laboratory. And what happens next depends only on your imagination.