Animal Crossing would be my bet.
Smash 4 Deluxe is up in the air and Pokemon and Prime 4 ain't coming any time soon.

An android virus that will brick all Switches that paid for online.

So new IP.

Animal Crossing

Unless Pokemon Switch is released in 2018 (but I doubt it).

I wouldn't underestimate Animal Crossing. It did sell 10+ million even before the 4 years later revision to include amiibo in the game (unless I'm wrong). Plus it's big in Japan, probably bigger than Smash there. New Leaf tends to keep popping in the Top 20-30 or so in the charts there even years after release.

It's a port of an already released game! (It's not going to drive hardware sales through the roof considering there's already a portable and a home premium version, DQ11 is not a big enough reason to get a Switch to rebuy the game again just because it lacks backwards compatibility with it's predecessor and especially when it's predecessor (3DS) is widespread as it is.) 

Would Yokai Watch's first HD entry be able to recuperate from it's conversion into modern AAA production values especially when it's made by a small player and is a declining franchise as it is ? (It'd be very hard for Level-5 to recover if the sequel didn't meet expectations and I noticed their alarming transition to mobile games since they have lower production values.) 

Nintendo should just port Super Smash Bros WIIU if it's going to be far off just like they did with Mario Kart 8 ...

Officially Metroid Prime 4 is 'TBA' and Pokemon RPG is '2018 or later'. My bet is Nintendo would prefer to have Pokemon out in 2018 as a major killer app but are prepared for it to be a 2019 title.

I've always thought Animal Crossing was a big contender for summer 2018 on Switch. Nintendo have hit the ground running this year with some big 'home console' franchises - Mario Kart 8 on the go, Breath of the Wild, Mario Odyssey. I did think from 2018 onwards we'd see more attempts to bring their core handheld market onto Switch so that the base keeps expanding, and Animal Crossing and Pokemon are easy ways to do that.

If Pokemon is 2019, I can see Smash Bros DX and possibly Metroid Prime Trilogy HD coming to Switch to plug the big, Pokemon shaped hole.

I was thinking Pokemon, but now they're saying it could be a 2019 title.

So, Animal Crossing, Smash 4.5 and Mario Maker 2. Combined with the new Yoshi game, Kirby and maybe Pikmin 4, +MHXX and DQ11 will prolly be Switch's biggest games next year.

DQXI port is not gonna sell 4M on Switch.

I'm just pointing out the power of the DQ brand.  XI will be a very significant release for the system in Japan.  Especially since it will be a portable version of the version seen on PS4, not the 3DS version.  This is almost an absolute certainty.

And Yokai is in decline but still very popular. It would be a regional boost though.  And they could create a game that is light-years ahead of 3DS production values and still not spend a ton of money due to the simplicity of the visual style the franchise aims for.  Whether Level-5 will do it, no idea.  I mean, Nino Kuni 2 (or 1 for that matter) isn't coming to Switch so someone at that company is dense.

As for a Smash port, I have my doubts since it missed this year.  

I would say next year the most likely candidates are AC, Smash or Retros game.For 2019 it will be Pokemon and Metroid.

The Animal Crossing team are the same team who develop Splatoon so AC can't really be out next year if Splatoon 2 has only just come out, the closest on that list to be out is Pokemon otherwise it's Retro's project.