Hard to say, with the baseline being so low in 2013, it won't be hard to improve by a fair margin yoy. The Wii was up about 50% in its second full year, once they got their supplies sorted but that was a whole other beast. But, seeing how low the baseline has been, a 50% increase doesn't seem at all unreasonable at this point, perhaps even a bit more considering that there are a couple of system sellers releasing and they will probably cut the price another 50$ in October or November, bringing it down to the Wii's original price point.
They will also keep on adding value through bundles through the year.
I'm guessing about 4.5-5 million sales for the year, which is 50-66% up yoy and lifetime sales between 9.5-10 million at year end 2014.
Oh, and games like Bayonetta 2 won't help much at all, I'll be shocked if it sells more than 300-400k lifetime. Below 300k would not surprise me one bit. X is a total gamble and can't be automatically relied upon to sell systems either. Wii Fit U will not be a factor.
That leaves Smash, MK, DK and possibly Hyrule Warriors, for Japan at least, to propel sales along with bundles, a price cut and advertising globally.
Those who expect 100-200 or even 300% yoy increases are out of their mind; some will argue, kind of like I did, that the extremely low baseline in 2013 will be easy to improve but I maintain that when a console manages to sell only 3 million or even less in a full calendar year and with no 8th gen competition until mid November, a price cut and a mainline Mario game for the holidays, the maximum potential sales are not very high; a software draught alone cannot explain such incredibly poor numbers, there is more to it than that.