The TheoryEditHmmm....lets think of a good game to start with...I know! Skyward. In the first level, Polvo makes a cameo appearance by Matthew's house. In Skyward's second level, there are snowmen that look like Schnee and an enemy from Water Sea Wars. So...maybe Schnee is part of an entire race of snowmen, and they all live in Snowy Spoundrum. Maybe all of Snowfall and the second level of Skyward take place in Snowy Spoundrum. But that isn't all for the Skyward stuff. In the third level, you can see a Muha Kid from Petit's Adventure and Bob from Cube Fighters. Maybe according to the creepypasta, the Muha Kid might just be Petit. Lets move on to Petit stuff. In Petit Lost Again, Desert Wolf, Tigzon, and Iguaba appear. And also The Mjduniverse Series is tied in, too. According to the page, Skyward, a new character called Professor Dunce is going to be in the series. Professor Dunce originated as a The Mjduniverse Series character, and he is Zack's stupid teacher, but he gets fired. According to his description on the page, he is Matthew's stupid teacher. So Zack is apparently Matthew's classmate. But there is more crossovers involving the Mjduniverse series. The really old games in The Mjduniverse Series feature Joe as the main character. So WKS has now been covered. But there is a character in The Mjduniverse Series called Citten, who has only appeared in a couple games. Citten looks a lot like Jessica from Catlorful. So maybe Citten and Jessica are related, but Jessica returned color to the world after it went black and white, and Citten became evil. In one Mjduniverse game, Zack meets Desert Wolf.
Desert Wolf 3 contains an easter egg/ secret world where he's in Petit's world, further connecting the tie between the Petit series and Desert Wolf series. Also in Tigzon, you can find easter egg crates that you can break. Once you do you could end up finding Sonic, Mario, or a Puffie. This is the first connection with a real game franchise. Also Religious2 announced a new movie/game called the Joe, Mike, and Nick story, connecting the Nerdy Nick, Why Kid's Shouldn't, and Blind Guy series together. Also Nerdy Nick had a crossover with Alex Ways. In Skyward you can find a Goron and a Chomper from Sonic, connecting their worlds.
In Jingo 2, Tigzon makes a cameo appearence along with a Puffie from the Tigzon Series. This throws Jingo into the mix of crossovers.
So we've covered Skyward, Polvo, Cube Fighters, Petit's Adventure, WSW, Snowfall, The Johnny Family, WKS, Catlorful, Desert Wolf, Tigzon, Nerdy Nick, Blind Guy, Alex Ways, MtPK, Jingo, and Kit Kat.
However, Petit's Adventure is only canonically tied to the Desert Wolf, Tigzon, and Skyward universes.
Tell me if I missed any hidden crossovers.