That was the time when Ichii was angry with Up Front for not inviting her So I agree, there's probably only those for missing:
- Eri
- Junjun
- Linlin
- Sayaka
(Though I'm not too sure about Sayaka since she did perform something for the 20th anniversary. Just not with them I think?)
Performing with Eri will probably be impossible
/edit: Forgot Konno. Which is sad because she has performed with other OGs while 9ki/10ki were in the group, too.