Every generation has third party hits that become really big.  Some come totally out of nowhere like Minecraft.  Others are sequels that break out from a more niche following like GTA2 -> GTA3.  For generation 9, I think these kinds of titles will be on the Switch.  

Hard to say which titles exactly, but I wouldn't be surprised at all if the next Monster Hunter title sold 10m+ on Switch.

I think it has a chance to have more than 10 ten million sellers, though I'm not sure what all those titles would be... Ones I see passing that mark are the next Pokemon game, Animal Crossing, Smash Bros Ultimate, and perhaps Splatoon 2. That's already 7. I think a few more can release in the Switch's lifetime...

Well, now that I think about it. It could have a better chance than I gave it credit for. The series managed to jump from 3.6 million with the first game to 5-6 million for the second game. I would absolutely love it and do jumps of joy if the game managed to pull it off because I love the Luigi's Mansion series, (my personal favorite Mario spinoff series and 2nd favorite overall right behind the mainline platformers). And for it to reach 10 million would pretty much guarantee that Luigi's Mansion becomes a regular, recurring Nintendo franchise. 

A new 3D Mario game could do it. It'll depend on two factors. 1) How late it releases in the Switch's life cycle, Galaxy 2 released in 2010 during the Wii's 4th full year on the market and at a time where Wii sales, while still impressive, were noticeably beginning to nosedive. 2) If the next 3D Mario is too similar to Odyssey like Galaxy 2 was to Galaxy 1 (It started out as just an expansion pack of Galaxy 1 before Nintendo decided to make it a full blown sequel), then it could eat into some of its potential sales.

And a Mario Maker 2 could certainly pull it off, or a new 2D Mario made from scratch. Either one of those could absolutely sell 10 million on the Switch.

The Switch will have between 15-20 10m sellers. I think any game series that's had a recent entry sell 4m+ on a Nintendo platform has a shot at 10m+ on the Switch as long as it's a good exclusive game that's advertised well.

To be fair, Wii Sport was bundled with almost every Wii console from Wii launch, so that hardly can be repeated.

Talking about potential 10m 3rd party seller, I would said potential GTA V could have best chance.

Yokai Watch sold more than 1M outside Japan (beinf released super late) I dont think it could pass 10M and idk if its still that popular in Japan but it could make great numbers.

As I read before I think Mario Party has a strong chance

Minecraft very well could do 10M. I don't think Just Dance comes back to this number, only if Nintendo pushes motion gaming again, which they seems reluctant to do.

Animal Crossing
Pokemon Let's Go
Pokemon (Newest version) 2019-2020
Pokemon (Remake or newever version) (2021-2022)
Some kind of sports casual game
Splat2on
NSMB Switch
Mario Maker Switch
Others that haven't been announced that appeal to the masses.

yea actually if im not wrong just dance 3 is the only 3d party to reach 10M in a nintendo system 

I voted 15 to 20.