Some thoughts I shared to a post made on the IGN Splatoon thread.
That last sentence boils down my view in a nutshell. Given how much Nintendo pushed Splatoon during E3 I don't see them shoehorning Mario in to increase sales. Maybe he'll be an unlockable surprise, I have no qualms with that.

Splatoon is out there and wacky, no doubt it leaves many scratching their head. The Wii did the same thing (even more so) when it was revealed. This carried forward to the Wii U and with the tidbits revealed for next-gen Nintendo I see them traveling farther down their own path, continuing to distance themselves from the other console makers**. The best thing about Splatoon is the amount of polish it'll receive before release.
I heard the devs say they hope to have it release in the first half of 2015 (I wouldn't be surprised if it falls to the second half, you know, Nintendo). Compare the Mario Kart 8 reveal to release. No bull shots, the game went nowhere but up.
** Or perhaps The Big N is on the same path they've always been and it's the other console makers who went down a different path.