My recommendations are:
1. Ayu
I don't know where you stop with Ayu... but seeing that you are a HONEY protector, I guess that would be around the MY STORY - (miss)understood era? Well, then check out Secret and GUILTY, her two newest albums, really awesome. Also, YOU HAVE TO listen to Mirrorcle World, it's like a musical orgasm!
2. Ai Otsuka
I don't know where you stopped listening to Ai either... but her latest single, Kurage, Nagareboshii, is a damn good summer ballad.
3. Leah Dizon
US-Import model... her vocals may not be very fantastic, but she has some good songs. Her 2nd album, Communication!!! is really good, and I would recommend from there, Without a good-bye, Lost at Sea, Love Paradox, Vanilla, Under the same sky and BxKxRxxx. Her debut album, Destiny Line, is so-so, but the singles are from there are pretty decent. Check out Koi Shiyou.
4. Yuna Ito
You might have noticed her with ENDLESS STORY and her role in NANA. Her 2nd album, WISH was out last year, and she did a duet with Celine Dion! It's called, Anata ga iru kagari ~A WORLD TO BELIEVE IN~. Nice ballad. Also WISH has some fantastic songs like I'm Here, Urban Mermaid, and her cover of My Heart Will Go On. Her latest single miss you, is one of my favorite songs from her, so check it out too!
5. Koda Kumi
Kuu is amazing, period. For starters, listen to TABOO, her latest single. It's a song that I know you'll like!
Take a listen to Kingdom, her latest album, as well. Nothing much to say cos she's so amazing! Perfect if you like R&B/pop songs.
6. alan
alan is a Tibetan... I think that's all I know about her. Her vocals are delicious, and you can hear that from her debut single, Ashita e no Sanka. Other singles (nope, no albums yet!) I would recommend are Kaze no Tegami and the Fire single... I can't remember the name =.=
Yup, so far these are the singers I listen to, so do check them out, because they are all amazing singers!