With both NSMB Deluxe & Mario Maker 2 both releasing this year, I have a hard time seeing a new 2D Mario releasing next year.
Odyssey 2 would be great but it has the same issue as BotW2 or Pokemon in that they will sell a ton of software and will move a good amount of hardware but their strength will be dimished by the fact that they all already have installments on Switch.
And I agree that we probably wont see a flat out price cut next year, added value bundles are more likely.
Overall I expect sales to slow down next year, not a huge decline though, maybe something like down 10-15% YoY so still very high sales.
I forgot about NSMB Deluxe coming out earlier this year. The thing is The Switch Lite will be available for all of 2020, while in 2019 it is only available for 3.5 months. Sales should be up year-on-year through summer 2020. Animal Crossing is bigger than any release in 2019, other than Pokemon Sword / Shield. I expect a lot more Japanese games coming to the Switch as it is the only platform that is still selling well there.