Wii U is definitely selling poorly but they will continue to support it for another 2-3 years. It still costs $300 and Nintendo has a bunch of 1st/2nd party titles that have yet to release. Mario Kart+Smash Bros+X+Yarn Yoshi+Hyrule Hyrule Warriors+Bayonetta+SMTxFE are all likely to release by this time next year plus potential unnanounced games like Zelda+Metroid+Kirby+Paper Mario+Animal Crossing+Kid Icarus+Star Fox+F Zero+Punch Out+Mario Party+Pokemon+NFC toy game and any other 3rd party collaborations.
Those games plus future price cuts wont all of a sudden make Wii U sell amazing but they can make sales increase enough to let Wii U be on the market for 2-3 years before getting replaced, 15-20 million lifetime is probably what Wii U will end up with and thats enough for Nintendo's franchises to sell solid numbers. After that they can release new hardware, preferably a unified console/handheld.
Release Nintendo Fusion in fall 2016 in two seperate forms, Fusion Home & Fusion Portable. Similar hardware with the ability to play the same software. Home version will be slightly more powerful than Wii U, kinda like Wii was to Gamecube and the Portable version moderately more powerful that Vita, kinda like 3DS compared to PSP. Each sku will sell for $199 and Home will come bundled with either a Motion Plus or Pro Controller, maybe even both.
Nintendo struggles to support 2 platforms at once but with a unified console/handheld there output would be great. If u look at 1st party titles released from Nov 2012-May 2014 its quite impressive, Pokemon Mystery Dungeon, Animal Crossing, Nintendo Land, New Super Mario Bros U, Fire Emblem Awakening, Game & Wario, Luigi's Mansion Dark Moon, Donkey Kong Country Returns 3D, Pikmin 3, Wind Waker HD, Link Between Worlds, Wii Party U, Super Mario 3D World, Mario Party Island Tour, Yoshi's New Island, Kirby Triple Deluxe, Mario Kart 8, Mario Golf World Tour, New Super Luigi U, Mario & Luigi Dream Team, Donkey Kong Country Tropical Freeze, Pokemon X/Y. We could easily expect to see at least one 1st party title month and since they arent spread across 2 seperate platforms they would only need to make half as many of there traditional franchises meaning they could revive old ip or make more new ip.
This type of setup would also ensure that Nintendo's home console would get all the Japanese 3rd party support that Nintendo handhelds recieve. Capcom/Square Enix/Namco/Level 5/Sega/Atlus/Platinum all currently have a strong relationship with Nintendo and this will likely continue. We would also still see family friendly games from western 3rd parties like Lego/Skylanders and maybe still see the occasional annual titles like COD/Assassin's Creed.
Realistic sales for this could be 15-30 million for the home version and 35-70 million for the portable version giving Nintendo 50-100 million units of hardware sold next generation. With those type of sales games like 2D Mario, 3D Mario, Pokemon, Mario Kart, Smash Bros would all likely sell well over 10 million each and games like Zelda, Animal Crossing, Donkey Kong over 5 million.
Nintendo Fusion would be the home for Nintendo fans+families+Japan+secondary console. Basically Nintendo just needs to make affordable hardware sold at a profit with a steady stream of quality software and they will be fine.