A commission for ResistanceKing01! Story/Description provided by him:

Mario and Peach managed to permanently defeat Bowser once and for all. But as a consequence, they get transformed into koopas akin to Bowser. Luckily, though, they remain their true good-hearted selves. So then, Mario and Peach got married and become king and queen of the Mushroom Kingdom, rulers to both toads, koopas, goombas, and other creatures.