The steam controller 1 already has wifi, its not the only controller that does this, the nvidia shield controller also does. The standard these and a very few other controllers use is called wifi direct, and basically uses an adhoc network to connect. This is seamless for the user.
I think it's unlikely that it's a controller, but the days of 2.4ghz only are long gone. Sure you can still cheap out on it, but it's not that expensive anymore. I could see a controller using a chipset that had it.
That's would be really neat for those of us with lots of wireless interference in the 2.4 GHz band. I can't even really use Bluetooth controllers for that reason.
I hope that's the one. I had to connect the Steam Controller to my Deck though the Dock to play something on big screen and my fingers were fumbling for the difference in button layout and number. A Steam Controller 2 with the same layout as the Deck would be an instant wishlist topper