A new stage was revealed on the Japanese Splatoon 2 Twitter account today. It turns out a Mystery Zone stage will join the two normal stages during each SplatFest. This Mystery Zone will be different for each SplatFest.
The Mayonnaise versus Ketchup SplatFest stage seems to have an Octoling theme. The teaser image for it has Marina’s icon surrounded by takoyaki. The map itself looks like the Octoling stages present in Splatoon 2’s campaign, where you are actually in an Octarian area and fighting against multiple Octolings to collect mini Zapfishes. I think it looks a lot like the stages in World 5 of the campaign, particularly Stage 27.
Splatoon 2 is available for the Nintendo Switch. The Mayonnaise vs. Ketchup SplatFest will run from 9pm PT on August 4, 2017/12am ET on August 5, 2017 until 9pm PT on August 5, 2017/12am ET on August 6, 2017.