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
My number 1, must have feature is for the system to be portable. Hopefully that goes without saying.
If I was to ask for a new feature, it would be some sort of off the wall, possibly stupid gimmick. The WiiU tablet controller, despite being a total flop commercially, spawned some really cool, asymmetric games. Nintendo is at their best when they are being creative with their hardware. They can make up for a lack of pure computing power by providing experiences that you could not have on any other system.
I do love me some unique hardware experiences, but I hate that it’s inherently difficult to port them. There are a lot of wonderful games that have been stranded on consoles like the DS, 3DS, Wii, and Wii U.