Here's a hint
I'm still messing around with NES Remix 2, and naturally, I have all the games unlocked. But some people may not be as tenacious, still noodling around with some of the tougher Dr. Mario or Wario's Woods stages, and I totally get that.
To help get you back into the game, Remix producer Hayashida just posted a new Miiverse entry, noting that there's a secret code to unlock "all the titles you haven't unlocked yet." The code is given through the hint "START TEST CLEAR," to be entered on the stage select screen.
If you want to figure out the code for yourself have at it, but the solution is in the comments on the actual post (and it's a sure bet someone will post in the comments below). I'm just glad it's not a $5 microtransaction (take note, EA, cheat codes are still alive and well).
Developer's Room [Miiverse]