1) The key is the successful implementation of the common OS/architecture strategy going forward. If developers don't have to pick and choose whether they're making a game for handheld or home console, but rather they just put out the game they want with different graphical fidelity levels (like a PC game), half the battle will be over.
2) Launch the new console and handheld, which can both play the same games at different graphical levels, at or near the same time. Unify the brand, like iPhone and iPad, and get the user base for NintendOS as high as you can as fast as you can in order to sell as many games as you can.
3) Forget third parties (for now). This is not to say that you write them off and tell them to get lost, but rather you need to operate on the assumption that they won't be there. If you can support NintendOS with droughtless software releases by yourself, the ecosystem will be successful and the third parties will come on their own. Do what you can to get them, but don't depend on them being there.
4) The launch needs to be a big event. The first six months to a year needs to be packed with releases that get attention. 3D Mario, Twilight Princess Remake, Splatoon 2, Mario Kart 9, Retro Metroid, an Amiibo focused game, a Rainfall-esque game, and a couple of third party collaborations like (but not necessarily) Hyrule Warriors 2 or what Sony has done with Street Fighter V to fill in genre gaps. The point being that people won't buy in on hype and potential alone. They need to look at the near future and say to themselves "Yeah, I need to play at least a couple of the games on the docket here." Throw in the next gen of Pokemon, playable on both handheld and home console, and watch adoption rates jump.
5) Iterate the handheld and console every two years, switching off so that there is a new piece of hardware out there each year. I'm talking upgrades like the difference between iphone/ipad gens, or upgrades like 3DS to New 3DS, all fully backward, cross, and forward compatible. With this system in place, the idea of hardware generations will be a thing of the past. Nintendo won't be starting from zero at hardware transitions ever again, and they can gradually keep up with advances in power while still catering to the people who don't want to upgrade quite yet. Just think of the game sales difference if Skyward Sword were available on 3DS and in HD on Wii U at launch, and if NSMBU were available to buy on the Wii and 3DS in addition to the Wii U. We're talking millions more in sales. Make this kind of thing standard going forward and both the software and the hardware is always relevant, competitive, and in the minds of the overall NintendOS userbase.
Edit: 6) I almost forgot an important piece. Virtual console. This needs to be up and fully running at launch, with at least the current full contents of the Wii U eShop available for purchase right off the bat. The entire Wii U catalog should be hitting new buyers in the face at a discount. Anybody who didn't buy a Wii U would most likely dip into that catalogue if it were sitting right there for the taking. This is a unique situation, because the Wii U catalogue will not have been experienced by a huge number of people who buy into the new ecosystem. Even up to 4/5ths of current 3DS owners haven't played a Wii U game.