An anonymous 4chan user has begun posting supposed concept art for a crossover between Splatoon and Super Mario Sunshine. The first picture depicts an Inkling fighting Petey Piranha, but the important part is that the Inkling is wearing the Hero Mode gear; the leaker claims there will be a new Hero Mode world with a Petey Piranha boss fight. The second picture shows Mario in a Blooper form, which could possibly be an alternate Squid form used if the player has any Mario gear equipped. The leaker claims that he'll be posting another picture "every hour or so," so I will make sure to update this post as he releases more.
The leaker has added more pictures, including a screenshot of one of the shirts that will be added in the update. They claim there will be a new game mode added after Rainmaker titled "Clean Up Crew," where one player goes around with FLUDD and cleans up.