Probably between 15-20 million. All this 25+ million is simply fantasy. After January the Wii U will be back to a 60k weekly baseline(maybe less). Holiday sales have been bad despite Smash Bros. and Bayonetta II did basically nothing for the hardware

4 million units a year seems extremely difficult, specially considering that it barely surpassed the 3 million mark in 2013 and also this year. But hopefully it can do those numbers.

Is almost at 3 m by 13th december, is going to reach around 3.5 - 3.7 m this year (it just need 200k per week this 3 last weeks, and we probably are going to see +300 by the 20th december week), that is better than the 3.1 m of 2013, around 20% better, is actually growing without a price cut and with the japan drop. If you think on some of the games that is going to have on 2015:

- Xenoblade X
- Splatoon
- Star Fox
- Zelda U

Any of these games could be a big bomb for Wii U and increase the sales as mk8, maybe the 4 can make it on different levels. Plus:

 -Devil´s Third
- Yoshi WW
- Kirby rc
- Mario Maker
- Shin Megami Tensei X Fire Emblem (if is not cancelled)

Those games are not going to make a boost on sales but they are going to keep the interest on the console along the year. 

Finally they are going to make a price cut somehow on 2015, that will make another boost. And I think wiiU is having so much better reputation than a year ago, I believe people is looking it with better eyes than before, it has become more relevant and some of this games may finally encourage many of them to buy it. So seeing how WiiU is slowly growing since its release and seeing the strong collection of games that are coming in 2015 and the price cut is very possible to see WiiU getting better sales next year , i´d say no less than 4 m, even 6 m is not impossible at all.

Very very wishful thinking here in my honest opinion. 6 millon in 2015 is totally not going to happen, even 4 is a stretch. X1 and PS4 have some hard hitting titles out next year and the PS4 has not even officially dropped its price. The Xbo 360 almost outsold the Wii U in the US last week which is the Wii Us biggest market, things are very very bleak for the Wii U. I can definitely see it selling well under the Gamecube at the end, easily.

Well, yeah maybe is wishful but i have reasons to believe it,  do you really think this has been the WiiU peak year? Do you really think is going to sell worse on the year of Zelda, splatoon etc.. and the price cut? No way, i don´t think so.

As long as no price cut wiiu wont go anywhere beyond 15m in lifetime. People talk about profits but does it really matter for a console to make profits if every game on it is made by nintendo? With a higher installbase more nintendo games could be sold, much more profitable.

I personally do not see 2015 eclipsing 2014 in terms of hardware sales. 2014 saw 2 of Nintendo's biggest and most relevant franchises brought over and sales were still lukewarm in general. Star Fox has not been relevant for years and Splatoon is not a system seller. Smash Bros. and Mario Kart are way bigger franchises than those two. Zelda could go either way in terms of pushing hardware as interest in the franchise has waned over each iteration. The only franchise I can see now pushing big hardware figures is a true 3d Mario akin to Galaxy/64 and that may or may not be on the Wii U and is certainly not coming out 2015. Just my opinion anyway.

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2014 saw 1 of Nintendos biggest titles as Wii U only release and 1 of their biggest titles seeing a 3DS first release. And this second title isn't that big in Europe.
So Zelda should have a much bigger impact than Smash Bros., a possible price cut will have some impact, depending on how big it is, and there's a ton of other exclusives that might not be big hardware sellers themself but should keep interest up as a whole.
A Mario Maker might possibly push Wii U more than people think and there's other games as well that could have some effectt in one region or worldwide.

So yes, 2015 could be a better year than 2014, especially if Nintendo has one or two unanounced titles like this years Hyrule Warriors (announced Dec. 2013) and Captain Toad (E³ 2014).

It could as well be worse though, if they have delays again, no pricecut...

loving some of the responses in this thread.

The one about the Facebook console was the best.

Indeed, they're very much possible, but it's far easier if you try early on. Nintendo didn't do much for 2 full years. They should take notes from Microsoft in that regard. They saw what was happening, and canned their stupid plans, and when that wasn't enough, they cut the unnecesary fat (Kinect).

Nintendo, on the other hand, saw this problem, then they did what they always do, they threw Mario at it, but evidently, that didn't help much. They should've, and I know this has been said far too many times, but they really should've removed the gamepad. In 2013, that is.