I think HARUNA might become a dancer since her original dream was becoming a dancer. She might also become an actress since she used to has a minor role in a movie, although this chance is slim.
TOMOMI might be a dancer too since she had the same dream as HARUNA. Also, I think she is qualified to become a MC since she's funny and active, and her voice is perfectly suitable for MC'ing game shows, for example.
MAMI might become a painter or designer since we all know she has a good drawing skill. Her drawings are featured on many SCANDAL's items such as T-shirts, stickers, keychains, etc.
About RINA, I'm not really sure because she has many different types of hobbies. She might become a photographer since she's very interested in photography. I can image how her photography style is. It should be vintage since she loves the 60's era. She might be a restaurant owner since she wanted to try serving people at a restaurant; but this chance is slim though. Another chance, becoming a piano teacher (this is not contradict with the topic title since piano teacher is different to getting into showbiz.) Anyway, if RINA could not find a job suitable for her, she could always become my wife.