One appropriate approach is to write a story from the perspective of their different kingdoms. For instance, write about how Bowser's actions in his own dark - themed kingdom might be misinterpreted by Peach in the bright Mushroom Kingdom. It could be a story about cultural differences and misunderstandings between their two realms, with a positive message about communication and friendship in the end.
An appropriate way could be to explore their relationship in a more platonic sense. Maybe write a story where Bowser and Peach have to team up to face a common threat in the Mushroom Kingdom. They learn to understand and respect each other's strengths and weaknesses during this adventure.