One possible plot could be that they start as friends and gradually realize their feelings for each other. They might face some challenges from their families or friends who are initially skeptical. But through their love and determination, they plan a beautiful wedding that brings everyone together in the end.
In this fanfiction, it could start with Sweetie Belle daydreaming about her future and Button Mash suddenly popping into her thoughts. They then start to interact more often. As their relationship progresses, they start planning the wedding. There could be a sub - plot where they have to raise money for the big day. They might organize a concert (since Sweetie Belle is into music) and a gaming tournament (for Button Mash). Along the way, they learn more about each other's insecurities and how to support one another. And finally, they have a grand wedding with all their friends and family present.