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
For me it’s backwards compatibility with the Switch. I wouldn’t upgrade if I couldn’t play BOTW or TOTK on it. After that just more powerful graphics so I don’t get stutters in some games.