An au where either dancing is the profession instead of being youtubers, youtubers are known for dancing videos, or it's just an au where a lot of people enjoy dancing as a pastime. Not just ballroom dancing though, break dancing, tap dancing, figure skating even.
Scenario one: You're preparing for a competition but one of your opponents catches your eye.
Scenario two: You were in love with (crush) and at one point you were going to ask to be their dance partner for a competition or something along those lines but too late, they have a partner.
Scenario three: Make it up