I love Eri and Risa. They can basically make anyone sound good, while holding something toward themselves in the process. I think they would make an ideal duo (besides GAM), and would also make a great combination with any other people.
Ayaka. Pandas. I've repeated myself so many times. I'll say it again. I want a Chinese Cafe again.
The best idea would be to gather all the dormant groups into one mega-awesome group.
More good harmonizers with great solo voices would be Yuko, Kei, Kaori, among others.