The best idea is to scrap the Wii U and start all over again... and this is coming from a guy with 20 Wii U games that loves the system.
The XB1 and PS4 have approximately 5-7 more years of life ahead of them. If Nintendo could get a system out next Christmas with a the power of a PS4, that means it has 4-6 years to compete directly with those systems, with an ability to have all of the same third party games.
Right now the Wii U is losing money. The cost of the system will go down and the install base will go up and the system may turn a profit in the near future. The problem is from a business perspective, "turning a profit" isn't good enough. If the best case scenario is making a little money, the system is still a failure.
Launch the new Nintendo system Christmas 2015 and launch the new handheld along side of it. The big launch game? Pokémon, which can link between the handheld version and the first home system Pokémon that is cannon and not a spin-off. You can get the home system for $350, the handheld for $200, or the combo pack WITH Pokémon for $450. Handheld can be used as a controller, but it is not mandatory, so developers don't have to get creative when they don't actually have creativity. However, this combo does allow for backwards compatibility with Wii U games. Any "Indie" game that is downloaded, so not a full retail product that is on disc as well, must have mandatory cross-buy and it costs the same as the game on the XB1 and PS4. This would give the most value to buying the game on the Nintendo system, essentially locking down the sales of downloadable games for Nintendo for anyone that has more than one system.
I for a long time was thinking a system that Nintendo's savior would be a system that is BOTH the handheld and the console, but I am afraid that would be underpowered and Nintendo would be in the same place they are now, so I changed my mind about that.
Finally, what about the people that own Wii Us? Will they be pissed they bought a system that only had 3 years of life? Do Ambassador Program 2.0 and anyone that has a Wii U registered automatically gets a bundle of Nintendo software downloaded on their system. This will be if you buy within 12 months of release to provide an incentive for early adoption. Give something like 5 NES, 5 SNES, 5 N64, 5 GC, and 5 Wii games for free, but let the users pick from 15 games on each system. To make things even better, these games have exclusive re-skins never to be available anywhere else ever again... so perhaps you have a character select in Super Mario Brothers where you can use Link, but this is a simple sprite swap so it isn't too much work for Nintendo.
I just solved Nintendo's problems.