Here's what Nintendo needs to understand: for a game to sell a system, it has to have its own identity.
What do I mean? Consider the NSMB series of games. The original NSMB definitely was unique at the time, a complete reimagining of 2D Mario. NSMB Wii then said "let's do something completely new" and added the active multiplayer. NSMB 2 was then NSMB with lots of coins... a bit of a new twist, but not much of one. NSMB U was then basically the same as NSMB Wii - some tweaks, but nothing to make it truly unique. Unsurprisingly, the former two both did exceptionally well, NSMB 2 has struggled somewhat but isn't doing too badly, and NSMB U is struggling quite a bit (it's not really selling the system the way it should have been able to).
Look at the Super Mario 3D series - 3D Land was a completely new Mario experience. 3D World was basically Super Mario HD Land. The former did quite well, the latter is struggling by comparison.
Donkey Kong Country: Tropical Freeze is basically a HD version of the previous title. So is Pikmin 3. And of course Wind Waker HD (but that's a more literal case). Wii Fit U and Wii Sports Club similarly are more HD upgrades than new games.
Mario Kart 8 goes against this recent trend - instead of just refining a previous title's gameplay, it introduces some completely new ideas - most notably, the antigravity. It makes some remarkable new tracks possible, and it causes the retro tracks to get some great tweaks.
It leaves me with some hope, but also a bit of concern. Smash Bros U looks like a refinement, and that does concern me a little. Hopefully, the appeal of the series, and the potential for much more stable online with this iteration, will help. Hopefully, it will also have some other new twists to give it its own feel.
On the other hand, Hyrule Warriors is definitely going to have its own identity. X looks like it's going to be its own experience, as is SMTxFE. Bayonetta 2 looks to provide Wii U owners with something different from what came before, so it might help, too. And hopefully, Nintendo have some new games to announce at E3 that also go this way.
So... yeah, in my opinion, that's what Nintendo needs to learn from Mario Kart 8 - the importance of giving a game its own identity.