Well, this is an interesting idea, and pretty unique, too. Canon has patented a zoomable teleconverter. You know, those things that effectively increase the focal length of your lens? I’ve never even considered that a teleconverter could even be a zoomable device and I’ve certainly never seen one before. According to Canon’s patent, though, it seems doable.
The patent seems to show adjustments from 1.0x to 1.5x and 2.0x with the movement of a lever, allowing you to quickly switch between a lens’s normal focal length, a 1.5x magnification and a 2x magnification. As Canon News points out, this offers a multitude of benefits from convenience to improved weather resistance.