Well, there could be a plot where Takashi and Saeko have to deal with cultural differences in their marriage. Maybe Saeko comes from a very traditional family, and they have to work through those expectations. Or perhaps a story where they are married in a magical world and have to use their combined skills to save that world. They could also face relationship issues like jealousy or misunderstandings that they have to overcome, which makes for an interesting read.
A really interesting plot idea could be that Takashi and Saeko get married in a world where their relationship is not accepted by society for some reason. They then have to fight for their right to be together. It could explore themes of love against all odds. Another plot could be that they get married in a virtual reality world and then have to figure out how to transfer their relationship back to the real world. This would involve a lot of creativity and problem - solving on their part, and it would be really exciting to read about.
One plot idea could be them building a new life together in a post - apocalyptic world. They face challenges like finding a safe place to live and protecting their new family. Another could be a story where they travel to different places and have adventures while being married.