I know it's only been 8 months since the Switch launched, but since 4K televisions are showing up more and more in people's homes, and because 4K gaming hardware will undoubtedly drop in price in a few years, I think the Nintendo Switch's successor should be made to do 4K gaming.
It would also need to be built around whatever major technology trends pop up around the time of it's release, provided said trends are gonna have a long life span.
The launch lineup of course must consist of high profile first-party Nintendo games that utilize the system's features best, as we've seen with the Switch launching with Breath of The Wild and Splatoon 2.
Marketing is also an absolute must, especially to avoid the mistakes Nintendo made with pushing the Wii U and it's software. The new system will need a big presentation to showcase it's hardware AND software, and be done in a way that makes people really want to get it.
As for when it would launch, I'd say around 5 or 6 years after the Switch.
Well, those are my thoughts on what the Switch's successor should be like, what about you? :)