Unless its this whole hybrid/fusion type of console, I see it as being their next handheld. Pretty much every major Nintendo handheld has released a year or two before a new home console, so I'd be surprised if they changed the schedule now.

Please be Fusion, please be Fusion, please be Fusion, please be Fusion...

I think you've got it wrong. NX is not a device but refers to a platform. Some kind of portable device and something that's used primarily at home, so 2 devices which share the same architecture and can both run the same code in games. Nintendo talked about this over a year ago, but it seems the information keen gamers of this world didn't hear what was being said, surprising really given how often gamers go on the internet about things like this.

Nintendo Xbox

NX is a line of hardware similar to Apple's. It will come in form of a traditional handheld, a tablet and a homeconsole. Maybe even the handheld will be in form of a tablet. Both will share the same new NintendOS and downloadable games/content will be pushed to be the main channel, same as Androir or iOS.

I just hope (if its handheld) that it replaces the DS line similar to how the DS killed the GB line

Could it be a 3rd pillar device? a bit like DS was with GC and GBA. Iwata has stated that NX isnt simply a direct replacement for 3DS/Wii U.

Still guessing the NX is a handheld

NX is a platform. There will be an NX Console and an NX Handheld that run the same OS, and development environments will be the similar enough to easily port games between form factors, regardless of hardware performance (Iwata mentioned that hardware power won't be a problem). However, not EVERY game will be playable on ALL form factors. Maybe there will be more form factors, handhelds with different power specs, consoles with different power specs, a tablet maybe.

No, it won't be a handheld with a dock, where the "dock" will have "extra" horsepower to make games run at higher resolution on TV. The "dock" will bomb worse than Wii U.

Guess one:
NX is hardware line that contains multiple form factors and shares the same architecture
Guess two:
NX is a chaep mobile SoC that will be mounted into diffent kinds of 3rd party devices.
Guess three:
NX is a modular console, a mobile motherboard that can inserted into terminal, like a cartridge, becoming an handeld a tablet or an home console device.