2012 launched with pretty decent games.

But I think Mario Maker could have been that one launch title that really could have gotten everyone interested and if marketed correctly could have even made people realize it was a brand new system :o

Splatoon would have a difference in how people thought about the Wii U, perhaps. It would have pushed people to believe that Nintendo was committed to doing new things with their software instead of just their hardware. I think it would have generated a lot more attention. I think even Zombi U generated attention for that same reason. XCX is kind of the same, though with a much more niche audience.

Of course, they still would have followed it up with a line-up of new versions for the same games they've been making for decades and that spark of interest would have waned once more.

Mario Maker is great for hardcore 2D Mario platforming fans but is it something most people care about? I don't think so. It's for the existing Nintendo faithful. I watched a video of a level someone made and I thought, "wow, that's creative," but I had absolutely zero interest in playing it or in designing my own.

things would have been brighter...

The games are aimed at Nintendo fans, which is great for them, and the reason why there are so many people happy with their Wii U (as their main console even).

But It will need to appeal to non-Nintendo fans to get more sales. 3rd party is the absolute key here.

So I voted 'done nothing'.

Things would be a little better, I guess, but not by much.

Splatoon is great and all, but 2014 with Mario Kart and Smash would be much better.

Same thing

I mean, yeah, it definitely gets the point across better, but I don't think the Wii U's problem was justifying the gamepad. No amount of gamepad utilization gets around the popular opinion that it's simply not an attractive controller, and that it's not as comfortable as a traditional controller. But in that case, 2014's line up is way better. Launching with Smash or Mario Kart would be much more effective at selling Wii U's than Mario Maker and Splatoon combined.

Would have definitely helped in a significant way. Maybe not 30 million units to date kind of way, but certainly much better than this. As much as I hate current-day Nintendo, I know I will buy a Wii U one of these days(hint price drop) just for games like Donkey Kong Tropical Freeze, MK8, Smash Bros, Bayonetta II, Yoshi's wooly world etc.

But you said that Mario Kart 8 and Super Smash Bros. would be both on Wii. 
I doubt that Wii U would sell good without both games on Wii U (even if they aren't launch games).

I think even just Mario Maker would have made a big difference. It's one the of the only games that really demonstrates what the Wii U is capable of. Unfortunately while we were all thinking about how the Gamepad could make the Wii U the best creative tool in the console space, it took Nintendo a few years to figure that out.