Since Nintendo announced they have a device in the works with the codename NX discussion is happening what this thing will be. Here are some option, you may add more in your posts:

Home console

NX might be a classical home console. Although it would be pretty early to replace the WiiU and it lefts open the question about the successor of the 3DS it is thinkable that the bad sales might lead Nintendo to the idea of early replacing the device.

Handheld

A handheld seems like the most obvious option. If NX releases late 2016 or early 2017 it would be the perfectly normal console cycle replacement for the 3DS. 3DS clearly is on the later half of it's life cycle now, a replacement in a bit more than a year isn't at all unreasonable.

Fusion/Hybrid

There are rumors about a fusion device for a long time now. Nintendo itself is participating with talks about unifying the platforms, although it is unclear if that means OS, software compatibility or a full fledged fusion or hybrid device. NX might be the implementation of such a device.

Quality of Life

A while ago Nintendo talked about a new device class they called Quality of Life. We didn't heard much about this in the last time, so the project might have been cancelled. But still there is the possibility NX is finally a first device in this class. The concept may have changed from the first ideas, maybe more integrating towards classical games (DQXI), but it still might incorporate QoL-ideas.

Software platform on other devices

It's a small possibility, but what if NX isn't a hardware at all? Maybe it is a software-platform for different devices. Think about the DeNA-deal, what if NS is a platform developed together with DeNA to bring Nintendo-games to other devices? Maybe it is some sort of mobage for home console and PC. This would allow DeNA to extend the reach of Mobage to more devices and give Nintendo the ability to control their games while releasing on a wide array on platforms.

Something else

Maybe you have better ideas? Post them!

Either Fusion or a Home console, I think SE accidentally outed it.

DQXI is also on 3DS, so all the other options are possible the same.

A Lexus.

That's why I think they accidentally outed it, as a handheld only the NX would surely have BC not needing a new version to be made especially when they could just do what MH4 does and improve performance and visuals when played on a more capable hardware only instead of N3DS with MH4 it would be DQXI on NX. 

In my opinion they not only outed it as a home console (although Reggie already KIND OF did that) they also may have confirmed that the NX is coming in 2016 because that is when the game is set to release. NX could be a holiday 2016 release. The only other option would be a delayed release for the NX and I feel like Nintendo already learned that the delayed releases really do not work when they tried it with the Wii U and everyone bought the games for the other systems first.

A dog.

Reggie and Tanabe also mentioned it is a homeconsole while Iwata said it could replace both.

It's a Fusion platform.

If the rumour that they want to ship 20 million in the first year is true, then it's a fusion.