I think NSMBU will definitely get there. RFA could though it's legs have taken a hit, so it might fall short by a few million. Switch Sports also has a shot but that really depends on if Nintendo is going to continue adding sports to it.
A brand new 2D Mario game should be able to break 20 million, but I'm of the opinion that they should hold off 2D Mario until their next system. Maybe around 2026. I know it would be weird to not have a new Mario game in the same year as the movie, but it's just as weird that a new Mario game wasn't even announced in the lead up to the movie. As a matter of fact, NO Mario games have been confirmed for this year so far and we are more than halfway through May. There is nothing on the horizon. Has that ever been the case?
They should hold off on 2D Mario because that is going to be a system seller. You don't need a system seller this late in the Switch's life, especially since NSMBU DX is doing a good enough job. Kind of like how you have MK8 DX and don't need MK 9 on Switch. 3D Mario at launch in 2024 and then 2D Mario two years later would be ideal for me honestly. And then they could do Mario Maker 3 roughly 2 years after that. I know that it would be 7 years between Mario Maker 2 and a new 2D Mario if we wait until 2026, but it's also been almost 6 years since Odyssey. Also, isn't the 2D Mario team working on Pikmin 4? I don't think the 3D Mario team has releasing anything since Bowser's Fury.