This generation did show that Nintendo can't rely on the casual market, unless they want a console that's only really viable for 2D Mario, Mario Kart, Sport minigames and rhythm games.
Differentiating themselves from the hardcore focused competition was definitely a good idea, even if it damaged third party relations. Nonetheless, they should have tried harder to keep 3rd parties happy. Perhaps more modern architecture was the way to go, but hindsight is 20/20. Wii looks to me like it was a console designed so as not to damage the company too badly were it to end up being Nintendo's final home console.
Content is king, and the broader your content the more appealing your product and the safer your strategy.
Lessons will have been learnt from the Wii, but at the same time the arms-race mentality has been damaging to the industry as well. An increasingly polarised industry. It's all about striking the right balance as well as ideally brining something new to the table as well.
It's a complicated recipe to get right, which is why Wii U fell flat last year as it was all half-baked. They need to do a lot this E3, but the fact it's been AWOL so long is good because they realise that. Nintendo need a eye on future-proofing the console but also presenting something that solidifies the whole concept in the way Wii Sports did. Easy to understand, appealing and something genuinely new. Chase Mii, Battle Mii, NSMB Mii, none of that cut it.
Also the one controller limitation better be gone and shown to be gone, as from a game design perspective you open up so many more possibilities. Things get much more interesting and exciting then, as well as far easier for people to understand the concept. Pricing issues and everything can be worked with, but it's making the Wii U's concept compelling and easy for people to grasp like the Wii which is key.