Best case scenario is it will be a Mario game with some great new mechanics. AND a new IP games that caters to the western audience.

But realistically I expect to be disappointed with their launch titles.

Zelda Wii U. :p

In all seriousness, it might be anything really. If it's the Fusion rumor is true. Then we'd get a console+handheld hybrid machine that can take advantage of the best of both worlds. A system where you can play games on the TV as a console and then take the controller with you anywhere you go with them and having it act as a handheld, and a fact that they would all be housed under one roof and share the same library would mean one franchise in particular olwould finally see it's true, long overdue debut on Nintendo consoles.
When you look at libraries for all Nintendo handheld and consoles, the one franchise that is THE system seller and killer app for their handhelds, which is sorely missing on consoles is...
Pokémon!
Yeah we've had battle focused games like Stadium, Coliseum, and Battle revolution, etc.
But we've never had a full scale RPG or Pokémon Generation appear on consoles.
Generation 7 as a launch title for NX (wishful thinking at this point since Gen 6 is about to rap once Pokémon Z eventually drops, and the 3DS has the established install base) or having Generation 8 later on during the system's life cycle would have people lining up the stores to buy the system!
You don't think the first ever full scale mainline Pokémon RPG on consoles AND in high definition wouldn't have people lineup up the buy the system?
I mean there are more copies of Pokémon X/Y out there than there are Wii U units sold.
The people over at GameFreak, Pokémon Company, and Nintendo have said many times before that they would've done it by now if there was a way to simultaneously create the handheld experience on consoles, well the NX Fusion system is exactly what the Doctor ordered in that regard.

As I've said earlier, I think Pokemon could be a killer app. I just don't think it realistically will be. The timelines don't match up.

I actually quite agree, but then the real question comes as 'is there a possability of a killer app anymore'? I'm not so sure there is. IF there is, it would likely have something new tied to it. Something that many people will call a gimick but will bring a new sort of experience. I think that's what many are thinking VR is going to be, I'm not part of that crowd though.

I'll stick with my original idea then.

I 100% think there is. Like there's no doubt in my mind. But there's no change of it being a "hardcore game." The barrier to entry for those types of games is too high.

Like I said, I think it'll be something super casual and super social. If Animal Crossing didn't exist, I'd be describing Animal Crossing. 

Splatoon 2 is something more likely. Something fresh(er) ... 20-30 year old Nintendo franchises aren't going to be a killer app to people who don't buy Nintendo systems.

Amiibo will be a central part of the NX like it or not too. 

It needs to be a mature shooter, that's for sure. If Splatoon can carry up a lot of hype, imagine what bloody Nintendo EAD (it's not EAD anymore if I'm correct) shooter could do.

Ok, no.

Super Mario 128 please!

I don't Splatoon has anywhere close to killer app potential. I don't think it'll be old IP either. I think it'll be something new, like what they did with Nintendogs, Wii Sports, and Brain Age.

Also, I don't think Amiibo will be used as much more than the replacement of physical media for the NX. Like it'll be used in games, obviously, but the "central part of the NX" thing will be relegated to Amiibo with download codes being sold at stores instead of physical media, and the eventual "Amiibo Land" game.

I don't think any shooter, especially something like Splatoon, could ever be a killer app. It needs to be something far more universal.