I always thought there should be another female Dragonball Z character. We have Chichi and Bulma who are noncombatants, Launch who vanished in Z, and 18 who doesn't even attack Super Buu.
I always thought Vegeta would be the type of person to have a sister; he's already got a wimpy younger brother so the opposite of that is a badass older sister or a twin sister. I am thinking like Luke and Leia Skywalker here. Would be funny if she fell in love with Goku and Goku's too confused (kinda like TFS Goku)
However, Goku could also have a sister but he already has Raditz and Turles (unofficial and we don't know whether he's his brother,cousin or uncle). Considering Raditz is dead, it would be better for him to have a secret kid. Maybe she could try take revenge on Goku and Piccolo but eventually become a Z fighter.