It's for sure no easy task. Wii U version would have to squander in abouyt 4.5 million more copies sold. However, Smash Bros continually sells pretty decently at retailers and amazon.com 

We can also most likely see a few things that will help it. First, being that there will probably be a bundle for it coming out next holiday, and that will make sales boost very nicely. Second, A price drop for Wii U is predicted this year, and It will probably drop down to 249.99. I know plenty of people in my town that are waiting for a price drop, then they'll get a Wii U. Third, being the general population of Wii U owners will have grown, there for more consumers to buy Smash Bros.

I think it will be hard, but not impossable for Wii U Smash to pass the Game Cube Smash. What do you think?

Maybe. I would say 50/50.

I don't think it will pass the original

Yes, is already selling faster than Mario Kart 8, maybe I will give 70% chance.

I wouldn't hold my breath.

I really don't know, maybe, but it all depends how much Wii U sells lft

Could happen, but it will be hard

It all depends on what sales are like in 2015. If they continue to stay above 30-40k weekly, and the holidays do it well, it should easily. But if they drop below 20k and are abysmal on holidays, no chance

It's possible, but I'm not going to make any judgement calls yet.

If Mario Kart 8 probably won't do it, neither will Smash Bros. Legs on Wii U so far have been mediocre, comparatively speaking about Nintendo's past titles.