Here is what I would like to see in Nintendo's next console. It doesn't have to try and be significantly more powerful than a PS4 or XBOXONE in my opinion, if they were to release a new console with specs very similar to the PS4 this would enable Nintendo to compete graphically and at 1080p but more importantly should help bring 3rd party developers back to Nintendo which they really need to make it successful.
I would opt for an AMD Custom 8 core CPU running @ 1.8ghz, AMD GPU capable of 2 teraflops and 8gb DDR3 memory to help keep costs down.
If Nintendo went this route, it would also make it easier for developers to code and port to as all the consoles would share the same hardware design, at the end of the day its not really the hardware that makes the console great or not, its the games made for it, and Nintendo have a head start on everyone in that department with a HUGE back catalogue and amazing IPs.
If possible I would keep it backwards compatible with Wii and Wii U games through emulation, again to keep costs down, and would use a Blu-ray optical drive and 500gb internal HDD.
Finally I would definately keep the Wii U gamepad but refine it slightly, making it slimmer, lighter and with a higher resolution screen.