This.

Create a portable and console version like the TG16.  Every physical copy can be played on either the console or the portable, if buying digital, one game per NNID can be used on one portable and one console.  Since it's the same game, there's only one development cost, no porting, online servers will function for both, etc. 
The console version will use the gamepad and tv for top/bottom screen, the portable version will be a DS like console.

Third parties would want to develop for the system because they'd capture sales from two different fanbases.  Portable console and home console  while only having to develop one version, cutting costs and cutting the amount of development time down.  Nintendo makes tons of money.  Win win for Nintendo and devs.

Pre-fucking-cisely.

Ain't that the truth.

Why would Nintendo make the same mistake twice? 

What mistake? They designed a very efficient piece of hardware with no bottlenecks that they can scale up and scale down( shrinking in the die ) in some years.

5m sales in 14 months is a pretty big mistake in my eyes. Nintendo needs to do something completely different next gen, something totally unexpected. 

Not from an internal hardware perspective.

Except that, for the umteenth time, the only big mistakes Nintendo made had nothing to DO with the hardware, but rather, a huge software drought in 2013, and a total lack of advertising for their console over that same span of time. That is where they fucked up, nowhere else. Honestly, when you say "they need to do something totally unexpected next gen", quite frankly, they already did that the last TWO gens. Wiimote. Gamepad. No one really saw those coming before they were revealed. Not totally.

Release the Wii U2 and package it with a free Bono fathead.

Thing would sell like crazy.

I'd say that the price also had a big role on the poor sales of the console.