Beside some of the ones mentioned, I only know
Hitomi Kaji. She hasn't debuted yet but she'll probably do soon. She was the winner of Tokyo Girls Audition in 2014.
https://www.youtube.com/watch?v=HCyVKuIZKBM
https://www.youtube.com/watch?v=fT6OSeSOQsg
It seems to me she's really influenced by Ayu...