I really love the slide-in controllers.
A few things I hope for would be:
* Open specs for the slide-in controllers so that third parties can satisfy various niches (like the fighting game community)
* A standard USB-C or micro-USB charging port so that people can use their existing power banks/chargers
* A well implemented eStore (with purchases that aren't bound to a single device) with a huge back catalog of games from their older consoles
* Compatibility with Nintendo's older wireless controllers