$350 / £300 with a killer exclusive packed in (think Mario 64). No stupid gimmicks.
For that money they can easily top the PS4 in performance 3+ years later. Not by much, just within the same ball park. Like the delta between the XB1 and PS4 is now. That's enough, anymore would be a bonus.
The way I see it they have two possible stances -
1) Are we happy with selling to the Nintendo faithful only?
2) Do we need to expand our horizons and get all core gamers interested in our product?
If they go for option 1, they can release pretty much any hardware they want with gimmicks galore, most of the Nintendo fans will buy it regardless. However that's all they will sell, and they will be looking at around a 20 million install base at best. I'm not sure how much longer that's going to be viable for them
If they want to achieve option 2, they have to take the core gaming market seriously. That means releasing good hardware and getting full software support for their machine. Even then, they will have a hard time persuading people to choose Nintendo over the competition. But with the right hardware and software support, they at least have a chance.
Releasing a console less powerful than the PS4, 3 years later, will only serve to put a big black cloud over the product before they have even started. Coming off the back of a failure with the Wii U, that's the last thing they need.
Nintendo are going to need very strong reasons for people to choose an NX over a PS4 or XB1.
With all that said, I'm not even convinced that 'NX' is a console. At least in the traditional sense.