They were actually bundled in with hardware for the vast majority of their sales. But that's beside the point, the distinction with those games is that they were selling millions upon millions of consoles by themselves.

An example of a high seller that is not a killer app: Mario Kart Wii - it sold 36 million units, but the people purchasing it were not buying the Wii specifically because Mario Kart Wii was on it; and therefore, Mario Kart Wii was not a killer app, just an extraordinarily high seller: third highest selling single platform game in history.

A game like Mario Kart Wii sold amazingly well, but very few people bought a Wii to 

Super Mario Maker 2 or TLoZ Maker.

Well it would probably be fire emblem for me. But for the rest of the gaming community I imagine it will be between Pokémon/Metroid prime 4 if one or both launch in 2018.

I don't think will see smash until 2019,but if we do see it release in 2018 that would also be in the run in and may very well be the killer app of 2018.

I'm not expecting Pokemon and especially not Smash next year.

Animal Crossing seems very plausible tho.

How dare you to forget Fire Emblem in the OP. I think that one will be the 2018 Killer App for the Switch.

Pokemon and Metroid Prime 4 both had nothing to show at all, so I doubt they will be 2018 games

Metroid, as cool as it is, is grossly overblown on internet forums for its sales impact.

It's firmly on the lower rung on Nintendo's mid-tier franchises.

Fire Emblem and Luigi's Mansion are both bigger IP. Not freaking chance would Nintendo ever let Metroid be their "big" 2018 Switch title ... there will be at minimum 4-5 Nintendo games that have far larger market impact than a Metroid game.

As someone who views Metroid as my favorite Nintendo franchise, this is mostly true. I wouldn't say it's on the "lower" rung, but it is a mid-tier franchise when compared to Mario, Mario Kart, Zelda, Pokemon, or Smash Bros. (and Splatoon, at least in Japan). It will certainly sell much better than something such as Arms, but it isn't going to light up the charts like a nuclear bomb the way a new Pokemon will. 

I would even neccessarily be so sure of that. ARMS has already shipped 1.12 mill or so, Metroid Prime 2 didn't hit a million and Metroid Prime 3 only got to 1.3 million LTD and that was released when the Wii was absolutely red hot (fall 2007). Prime 4 probably tops off at about 1.5-2 mill LTD though it could be helped by the Switch have a more core-centric userbase. 

I get where you're coming from and you may be right, but that core-centric userbase is why I think Switch owners will be much more receptive to Metroid than Wii owners were. 

It's still kinda the same userbase though, Metroid Prime on GameCube had a ton of hype and still only topped off at around 2 mill.

The truth is Metroid is just too slow/solitary of an IP to really have a huge mass audience. It's always been more "Alien" than "Star Wars".

Nintendo would be wise though I think to make a new FPS franchise that they own. Splatoon is a third person team based shooter and that's done great, but I think there's still a void that Nintendo could fill with a multiplayer FPS shooter, GoldenEye and Turok and Perfect Dark did so well for them back in the day, I think leaving the genre and/or thinking Metroid Prime was somehow going to fill that void was always a mistake.