By far the most important thing for me:
Keep the wireless seperated controller. Some kind of WiiMote and Nunchuck or the Joycons has become a MUST for me. I can not play with an regular gamepad anymore. It is just a pain for me, the Joycons are sooooooooooo much more comfortable. I never want to miss them again. I dont really need them for motion control, but I need them to have both arms independend from each other.
But they need to make the Joycons much better. The connection range is really shitty sometimes. When there are some obstacles like a couch and a pillow the range goes down to not even 1 meter - when I moved some furniture I really had a problem with that. That was really annoying.
Other than that I dont really care what their next system will be.