Here's my prediction. Post yours in the comments.
Wii U had a tough first half of 2014. Selling nearly as bad as 2013. Mario Kart 8 had come and has boosted Wii U sales for 2 months now and has had a steady baseline for awhile of 60k. If this keeps up it should hit 7 million in about a month. Which is going to be the very beggining of September. When will it hit 8 million? By September Wii U baseline should drop just slightly to 50k-55k. Probably will sell 250k because of Hyrule Warriors at the end of the month. Puts Wii U at about 7.3m... Now lets go to October.... Probably will have a very small boost to baseline.. selling about 65k to maybe 70k. So lets say 67.5k. Now with Bayonetta 2 at the end of the month.. it will give it a small boost. Wii U will probably sell just short of 300k in October. This puts Wii U at 7.6m... Now November.. it has 5 weeks.... Smash Bros will probably release in mid-late november. Along with Sonic Boom and Captain Toad Treasure Tracker. November I think will top 1,000,000 because of this. Maybe even a bit over. and Black Friday and getting close to holidays. Wii U will have sold 8.7m... December.. the largest month in terms of sales. Probably will be 2,000,000. Putting Wii U at 10.7m sold..... There's not a doubt in my mind it can't hit this. It did nearly 1.76m in Dec 2012 and 1.14m in Dec 2013.
EDIT: I forgot about Amiibo.