If you go by the first three generations, and take out the girls who already released a PB and/or (joint) IV, the following girls have the most 'right' to get a solo release of some sort:
- Urano Kazumi
- Akimoto Sayaka
- Umeda Ayaka
- Kasai Tomomi
- Sato Natsuki
- Masuda Yuka
- Katayama Haruka
- Tanabe Miku
If you cross reference that list with the preliminary senbatsu result, the top 5 looks like this:
- Umeda Ayaka
- Kasai Tomomi
- Masuda Yuka
- Akimoto Sayaka
- Katayama Haruka
In the world of idols, we'll most like see Itano's 6th PB before we'll see Umechan's solo PB, but I wanted to see which members have been passed by over the years.