Here's my idea. In the Super Mario Galaxy 2 universe, after Bowser disappears in the black hole, he somehow manages to end up in Equestria, more specifically the Everfree forest. He later finds discord, who proceeds to explain that he brought him here. He says They're both technically after world dominance, both want the princesses, and both have someone, or someponies, stopping them from doing so. At first bowser is suspicious, having experience from teaming up with villains in the past, but after Discord shows he already has Peach, Bowser jumps on board. Meanwhile, Mario and Luigi are frantically searching for peach, after reading a letter saying that "Bowser" took peach again. Being so soon after bowser was just disposed of, they used the starship to look for her. After a while, they finds Rosalina's Spaceship, and they notice another woman. She explains she's Princess Luna. She tells how she found out that Discord had broke out again, and that they had discovered that an unknown creature had been spotted in the forest. The following night, she finds out Celestia is gone, and there was a letter on the floor of the entry way, marked with some evil mark. Luna also noticed an orbiting station of some sort in an uncharted part of the night. After visiting said station, Roselina and her realize what was going on. Mario and Luigi decide to help their world, and Luna takes them back to Equestria, but what Mario and Luigi DONT know, is what they're about to become. Literally. How's that?