The closest comparison to Splatoon I can think of is Plants vs. Zombies: Garden Warfare. A cartoony colorful multiplayer third-person shooter with team-based combat.
Sales on 160 million (X360 + PS3) install base: 1.25 millions.
Sales on 30 million (X1+PS4) install base: 0.75 millions.
So far Wii U have only nine games over 1million in retail, most of them relying on well-known Nintendo franchises.
I think it will be hard to get sales over one million for Splatoon.
Although it will be a good lesson in online gaming (along with MK8, Smash and upcoming Devil's Third) for Nintendo.
Mario Maker can be somewhat compared to LittleBigPlanet 3 as they both are plattformers with cartoony graphics and are reliant on user-created content. Sales on double the Wii U install base is around 1 million in retail.
