You're right, no one's a perfect fit ('cept for Rika, Yui and Erika of course XD). Well, I still think Umeda would have actually fit quite well, she's tall, of a good age, and hey, having the same name as an ex-v-u-den member can't hurt either
Gaki does change her hair the moste often out of the current line-up I think XD She's no Ohtani but yeah. I kind of wish one of the Momusus would go blonde... I miss having a blondie in the line-up xD Maybe Eri, she already has the lightest hair out of everyone, I do believe.