Link?

I don't think we have enough information at this point to answer that question...

PS: Everything I heard pointed to the fact that there will be a campaign mode

We don't know that yet. Plus, I wouldn't say AC has a ton of variety. Content? yes. Variety? No. (That being said, it's one of my favorite Nintendo IP's)

Splatoon will eventually join Mario Kart & Smash Bros as it does what Nintendo does best...multiplayer. That alone will make Splatoon appeal to a wide audience. It will be big, and get even bigger next gen.

Eh, Nintendo will join Microsoft on the Mt. Olympus of multiplayer. Joy to the world!

I haven't played AC actually, but aren't there a bunch of little things you can do and minigames and such?

http://www.ign.com/articles/2014/06/24/splatoon-the-origins-of-nintendoas-adorable-squad-based-shooter

"A single-player mode is also in the works for people that can’t connect to a network."

Well they say "but multi player will not be its only focus" before that.

I have to agree with Sound Of Rain's optimistic view. The only thing holding this game back is the Wii U platform itself but it looks fun enough that it will sell well despite it. I sure hope Nintendo advertises this one.

Less than 1 million.

Between 1 million and 2.5 million.

I hope the game sells well, say +4mil, games like this needs to be a hit. But of course, hope is different from reality.