If Mario Kart 8 in particular fails to noticeably improve hardware sales for a sustained period, that would be the biggest dagger, in my opinion.
2014 is definitely the make-or-break year for Wii U as to whether it can even be a viable platform for Nintendo. Gamecube made them money despite its low sales…if Wii U can achieve that then it doesn't have to be a "failure." At the same time…to some it will ALWAYS be a failure because it won't match the success of the Wii or even come remotely close. So it's all really a matter of perspective and how you define things…it's very subjective. To me…as long as Nintendo can eventually make money on Wii U hardware and sell decent amounts of software and continue making great games that I enjoy (and hopefully ink out 20 million Wii U's in its lifespan) then it won't be a failure. Certainly not a success…just as Gamecube wasn't a success. But not a failure to me.
But to the OP, MK8 is the most important I believe…followed by Smash Bros….and 3rd (in my opinion) is actually going to be at E3 when they show off Zelda. I'm not one who cares about graphics or art styles and can appreciate Ocarina of Time, Wind Waker, Twilight Princess, and Skyward Sword for their wonderful and unique art styles equally (even if I do have personal preferences, of course)…but that being said, I really think showing off a more "mature" styled Zelda closer to TP or the Wii U Zelda tech-demo could have a very positive influence. Maybe not on sales immediately, as the next Zelda probably won't come until 2015, but in the opinion of the "gamer-culture" at large I think showing off an appealing-looking Zelda title at E3 this year could prove to be a VERY good thing for public opinion on Wii U. Now, if they show something more artsy like WW or SS, or something completely out of left-field art/graphics-wise, of course that has no bearing on the fact that it will probably be a wonderful, large, open-world, free-dungeon-ordered game (a'la A Link Between Worlds), but I do think that would only be another missed opportunity that could sink Wii U even faster.
Upcoming Games To Get
Definite: Kirby Star Allies (Switch), Mario Tennis Aces (Switch), Fire Emblem (Switch), Yoshi (Switch), Pokemon (Switch), Kingdom Hearts 3 (PS4), Monster Hunter World (PS4)
Considering: Fe (Switch), Donkey Kong Country Tropical Freeze (Switch), The World Ends With You (Switch), Ys VIII (Switch), Street Fighter V: Arcade Edition (PS4), Kingdom Hearts 2.8 Remix (PS4), The Last Guardian (PS4), Shadow of the Colossus HD (PS4), Anthem (PS4), Shenmue 3 (PS4), WiLD (PS4)