In this AU, the Versal Nexus is an interdimensional space with thousands of floating crystal spheres known as World Orbs. Each Orb houses it's own pocket universe, and together they form a multiverse of sorts. Many Worlds are based on different settings within various games and fandoms, but some are very closely related with only minor variations. Your character is whatever version of Princess Peach you'd like to create, and mine is Bowser in a quasi-human form who comes from a world where King Mario is the villain. We can hash out the details via PM, but basically my character hops into your world and recruits you because you have the last uncorrupted Mushroom Kingdom Heart, a glowing red crystal that exists in every world where a variant of you also exists. King Mario has been sending his minions throughout the Versal Nexus, hunting for all your variants so he can corrupt their Hearts because he believes that if he can sacrifice all the Peaches at once, he can revive his own world's Peach, who died some years ago while she was pregnant with his child.

Because your Peach's Heart has not been corrupted yet, it can cleanse other Hearts, and so Bowser asks her to set out with him on a journey across the Nexus to rescue the other Peaches and then find them temporary homes in other worlds until King Mario is defeated. Though, they don't all have to be named Peach, they could have variant names and such. We can develop the idea further via PM.