I think Smash will end up selling about as well as Odyssey. They hype for Smash is gonna be huge, the user base is gonna be way bigger when it launches, and normally it sells much better than 3D mario games. But Odyssey is also selling much better than 3D Mario games, and it really introduced new stuff to the mario franchise. I don't see how Smash will differentiate itself from previous games in the series. Personally, I'll probably end up buying it but to me I felt like the franchise didn't have much room to grow after Melee cuz it was already pretty much perfect. Still though, its a super popular party game and will sell tons, but because Mario Odyssey is blowing away sales records for 3D Mario games I think it'll be on about the same level, which is to say it'll be a 20+ million seller.
I'm assuming the Smash release will coincide with the paid online subscription debut. So the online setup for the game better be wayyyy better than Mario Kart is all I can say.
In terms of yearly NPD sales, I feel like Nintendo needs more than one big game this year to beat PS4's 2015. But it'll probably be close.