Unless they want a really long gap between the Switch's launch in 2017 and a Switch successor's launch, it has to be coming out fall of this year or by March 2022.
I think it will get announced this summer in some random event or possibly a Nintendo Direct.
I suppose there's also a chance that this piece of hardware is the Switch's successor, and they take the Sony/Microsoft approach where a lot of first-party titles are cross-gen for the first year or two. That way they can extend the life of the Switch while also bringing a full-on successor to the plate that gets a new audience. But for that to happen, I think the new hardware would have to launch in 2022 or later.
Lifetime Sales Predictions
Switch: 144 million (was 73, then 96, then 113 million, then 125 million)
PS5: 105 million Xbox Series S/X: 60 million
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima