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Toki no Sabaku - HnS style, but shorter and more emotional. The lines are perfect for Keiko, while Kaori could do the choruses and Wakana could do the ending "lalala".
Hoshi ga Tokete - Chiba Saeko's best song, at least for me. Again, Keiko and Kaori could team up for this one, and an ambient arrangement would make it even more sad.
Ballad - perfect for Keiko. 'Nuff said.
Shinkai no Kodoku - Yuuka's voice being similar to Kawashima Houko's, this would be a great song for her to do. It's basically fine as is, too.
Hoshi ga Kanaderu Monogatari - a lovely little song that deserves a Kajiura-sensei arrangement. Yuriko's style of singing would fit it very well, and a string quartet would give it a rustic, intimate feeling.
Solitude by the Window - I've heard a French vocal cover of this, and it's gorgeous. Wakana's Earth Song-esque singing would be wonderful.
Sayonara Solitaire - another beautiful Saeko song. Keiko's warm and sad tone would fit in the lines, and Wakana's more dramatic tones would fit in the chorus.
Egao no Wake - perfect for Kaori. 'Nuff said.
Fields of Hope - I've dreamed of hearing Keiko sing this.
Koi no Kiseki - a good song, but a little rambling in the original, and also doesn't benefit from Saeko's weak voice. Again, Yuriko would be a good choice, while Wakana could take the English parts.
Fuyu no Seiza - one of my favorite older songs from Kajiura-sensei, and deserving of a remake with Yuuka.
June Wind - yes, yes, I know it's completely random, but I want to hear Korechan do this one acapella with the girls in the background!