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
Next to basically everything that has been mentioned before (most notably backwards compatibility for games and the combination of handheld and docked), I would love it if the controllers would be backwards compatible as well. My joy-cons have the dreaded drift, but it would be nice if I can keep using my pro controller.
Reading these comments, I think the opinion of the community is that Nintendo got a lot of things right. Keep those, but just update it. Meaning more powerful and more reliable hardware.
The games are great, let us keep playing them and the new ones on the same system.
The option to choose and switch between handheld and docked is amazing. Both this and the previous point are what helps Nintendo set itself apart from the competition. Keep or even build on that.