Stefan could be really interesting. Since he has a long history with both Elena and Damon, his reactions to their marriage would be complex. He might be a bit jealous at first but then come to fully support them, adding an emotional depth to the story.
Caroline would be an interesting character. She's Elena's best friend, and she would probably be super involved in the wedding planning. Her over - the - top and energetic personality could bring a lot of humor and chaos to the story, like when she argues with Damon about the color scheme of the wedding.