This is a weird one, but I’d love to see a reimplementation of the sensor bar. The gyro controls haven’t cut it for me. I like how the Wii U game pad had the sensor bar built in to the bezel. The real engineering nightmare would be squeezing IR cameras into the top of the Joycon
Other than the obligatory backwards compatibility (I think if Nintendo screws this up again it would REALLY show in sales numbers), I would love the already paywall’d multiplayer feature to be somewhat useable on the next console (MM2’s multiplayer mode for example is like a bad joke on the switch).