Now, I think there are a lot of things wrong with the plot of BoF (mainly the boys treat Jan-di like crap) but its still funto imagine it as a less problematic drama.
Here's who I'd cast!
Gu Jun Pyo - Suga
The leader of the F4 and heir to the Shinhwa Group, one of the biggest companies in South Korea. He is hot-tempered and believes that there is nothing that money can't buy. Though he's unable to express his feelings, deep down, he has a heart of gold. He torments Jan-di after she stands up to his bullying ways, but ends up falling in love with her and will do anything to protect her.
Yoon Ji Hoo - V
A member of the F4 and the grandson of a former president of Korea. He has a driving phobia after being involved in a car accident that killed his parents and left him the only survivor. He is initially in love with his childhood friend Seo-hyun, who helped him overcome his social anxiety. Calm and gentle, his musical talent catches Jan-di's attention and she begins to develop feelings for him.
So Yi Jung - Jungkook
A member of the F4 and a skilled potter. His family owns the country's biggest art museum. After losing his first love to his brother, he becomes a Casanova. He eventually changes his playboy ways when he realizes that he likes Ga-eul.
Song Woo Bin
A member of the F4, his family runs the country's largest construction company, which Woo-bin later admits has strong connections to organized crime. However, he has excellent business sense and is very charismatic. He's a super nice guy that happens to be a gangster lol
and of course RM, Jin, and Jimin are the dad, mom, and little brother :)