An offshoot of the other thread on the best selling in the franchise this one is more on the opinion and predictions of which games could possibly cross the 20m barrier, me personally:

MK8D

I think it's certain at this point this will cross the 20m barrier even with being an enhanced port, it's previous version hit 8m on Wii U while MK7 on 3DS hit 17m the franchise is pretty much rock solid and LT I can see it easily hitting 20m.

SMO

Like with MK8D this game is pretty much a go to flagship title for the Switch so a large number of people in future will still pick this up helping it hit double diamond (diamond is a term for 10m sales in the music industry).

Pokemon

This is one of my main predictions, the momentum of the platform and high selling power imo can see the Switch title hit 20m and now the franchise has gotten an even bigger boost with former Pokemon Company head Shuntaro Furukawa taking over as Nintendo's new President.

Smash Bros.

As a major flagship franchise for Nintendo since its arrival I can see the boosted momentum of Switch and the combined userbase allowing it to hit 20m lt, a consistent 10m seller since Brawl and S4 has hit 14m total heading to 15m as the financial results have shown.

It will be interesting to see if Pokemon or Mario Odyssey does it first.

All 4 games will reach 20m, although it will take Odyssey and Smash some years to do so.

I also think Zelda can get there too. This quarter its attach rate was more than 50%, and that’s out of the holidays. It proved again that its legs will be on another level compared to other Zelda games. 

I mean, it sold 10m on its first year already...

Damn, I was expecting a serious discussion about strategies Nintendo could use to reach the lofty 20 million goal they set out for this fiscal year.

Splatoon could be a contender too. In more or less a year it has reached more than 6m and it is surely going to reach 10m. 20m might be a stretch, but if Nintendo is going to push their online games as much as I thing they're going to, Splatoon is going to be one of their crown jewels for a long time.

I'll ask that question after the system is at least at 40 million sold.

Yeah those 4 games will definitely all hit 20 million. Zelda will hit 15 mil and Splatoon 2 will hit 10 mil.

I think Pokemon is a given as long as Game Freak doesn't create something terrible. What concerns me is the total number of Pokemon at this point, and the amount of work it's going to take to model and animate all 1,000+ of them.

Although I've never completed the Pokedex in a single game, I love the idea of it being possible to do if I decided to put the time into it. What I fear will happen is that only a fraction of the Pokemon will make it into the game, what with time constraints, etcetera. This might not be a big deal for some, but it would be very disappointing to me. At some point it's inevitable, right? If they keep adding more and more Pokemon, at some point they will need to completely reboot the series, or...

Wait a minute... Could Pokemon Switch be a... nope, not gonna say it... :X

Same here

MK8 and SMO are pretty much a lock at this point, especially if they're bundled with the system for a period of time. Insufficient information available to determine the trajectories of Pokemon and Smash at this point imo.