Another new female solo is milet, another singer-songwriter type.
She just debuted (successfully) under Sony with her single inside you digitally.
Her first EP called the same is released 03/06.
If you're not into singer-songwriter or anime song singers like Aimer or Aimyon you will have it hard finding somebody new. Cause that's what's popular right now.
Not another Namie.
Or else you'll have to look for the old ones, still enough very regularly active (ayu, Kuu, Miliyah Kato, Mika Nakashima, Ai Otsuka, Nana Mizuki, Mai Kuraki, miwa...)
The closest new one probably is Chanmina, an urban JPop artist.
She made some waves with her two debut minis, Miseinen and Chocolate.