Ubisoft put in a patent application back in 2017 that details a modular game controller accessory system that's meant to be reflected by in-game elements. The patent application included drawings of the system snapped onto a Sony PlayStation 4 controller, along with what seems to be some sort of rudimentary VR controller. In both cases, the accessory seemed to connect wirelessly to the controller; the patent application's text does not go over how it connects, only saying that it can be held in place with physical brackets and tabs or by friction, depending on the controller it's mounted to. The system, once connected, can modify in-game elements based on what's attached to it.
In the patent application, Ubisoft details the fact that the base for the modules may come in a number of forms. A spaceship seems to be pictured in the drawings, but the company stated that it could be any number of things, such as a car, a boat, or simply a platform to put modules like figures and equipment on. Most of the changes to in-game elements seemed to center around the in-game avatar, which likely means that this idea represented a sort of toys-to-life gambit in a more detailed package. Players would presumably be able to buy and attach things like new characters a la Skylanders, parts for cars and ships, and modules representing other in-game items like a ship's crew. Essentially, it would be much like Nintendo's Amiibo system was with the Wii U, except that the toys would stay attached to the controller during play, and could be hot-swapped or added to at any time.
Ubisoft's patent didn't specifically go over add-on modules that could add in new gameplay features and control methods, but it seems well within the scope of the patent. Even without that capability, a unique sort of toys to life apparatus that attaches directly to a game controller could be quite interesting, especially in the context of mobile gaming. There have been attempts at somewhat modular controllers in and of themselves, but they never really caught on. This could be the next best thing. Nobody seems to be actively working on any publicly announced or even leaked products using this patent as of this writing, but the patent is somewhat recent and may yet see the light of day.