More than likely just a fake like Rayman stuff on Smash Bros but we can all dream about it

Seems possible... But a little unlikely. It seems like Nintendo want to keep crossovers out of it so whilst we could easily see levels inspired by Sunshine an actual section dedicated to it is a little out of reach IMO.

That....would be kinda cool actually.... Though he's not a kid or a squid! O: How would that even work..???

Splatoon is a sequel to Mario Sunshine. It's in the ''hero is defeated'' timeline where he loses during the battle of Super Mario Sunshine and the world is covered in Ink/paint until it eventually mutates into what we see now in Splatoon,Inklings. 

The Mario that succeeded in the ''hero wins'' timeline is told he must go back in time to change the past. With help from Professor Gadd,Mario builds a time machine which sends him back in time to current Inktopolis. 

Armed with his Fludd and his plumber wits, he must figure out how to defeat this new mutated foe

Perhaps he's the villain, trying to wash away the ink that is so important to the inklings?

If it does work I bet Mario takes some squid power-up to transform :o

That'd be cool.

Too bad Mario ain't a kid, nor a squid.

Do you have a link? Pretty please?

But Splatoon is set centuries in the future.

Mario's illegitimate offspring to a sexy female blooper secretly populate the worlds deepest oceans. During the Splatoon age, one particluarly adventurous Bloopario boy goes to the surface. He finds an old family heirloom FLUDD, amidst a world now covered in ink. It's time to get cleaning...

I wouldn't count it out. Mario characters were the original characters brainstormed for Splatoon. I would buy it. Seeing FLUDD spray ink would be totally ironic tho.