I pictured an art nouveau style poster from your sketch! Since its a dancing event, I think you could add more gesture into the people's poses. Right now its a little hard to tell its swing dancing because their poses are a bit stiff. Maybe try poses that show off more of that swing energy (example, one of her arms may be up in the air, head thrown back a bit, or her legs swing up, etc.) I would google some swing dancing pictures as reference for your poses. . If you dont want to change the people then maybe use more curvy wild vines to give the piece some movement. Good luck!