I feel Yuki Kajiura clearly loves Kaori.
Yeah she's definitely fond of her, having had her since just out of her teens, same with Hikaru. There's a bit of a motherly tone from Kajiura with the both of them, in my view.
I mean Kaori was the only person that could say during a live MC that her "fuin" was the first song they sang at the start of the concert! (referring to the "Anti-hero" theme for my beloved Kanagawa leg of the YKL#20, and Kajiura even made the joke funnier! [A bit of dark Kaori for those that know.

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I believe it's not always up to Kajiura
I see you on this point. Bigger factors in play and realities of the business, beyond just fondness.
someone (be it on Keiko's or YK's side) thought the collab was a good idea and a good/easier sell (for whatever reason, first anime single, ex-Kalafina singer and ex-Kalafina producer etc).
Yeah, that was brought to life. It's possible Keiko spoke up for it with her management, to do a fully produced track by Kajiura.
there weren't that many solo FJ songs outside of Yuuka before the lives started full force (and only Kaori got more than 1 song after Yuuka) AND all the FJ collaborations were WITHIN SPACECRAFT afaik...
Yeah Kajiura has shown she was willing to do more single tracks with a wider range of single solo vocalists, post-See-Saw, before FJ.
The Yuuka years were more of a showcase of a single vocalist and more akin to a Chiaki-lite (similar to the vibe with See-Saw) in my view, but with a protege rather than an equal.
Kajiura has been evolving as an artist and live performer over the years throughout her career, and our ladies were just along for the ride. As time went on, FictionJunction slowly became a blend of her solo-artist and group-live dynamics (with the grouped singers under her taking over for a while) and leading to the concretizing of her experience as a philosophy of the junctions of fiction, with the "Parade" album (and graphics) emerging as a final solidifying of that concept, audibly and visually, merging all three strains (guest solo, regular solo, and group songs) in one album. A picture of things to come going forward?
Kalafina took a lot of our her - and them - as a creative outlet. They were essentially a hybrid scenario: as a collaborator they were a single artist (made up of 3 singers), and as a group she worked with them as she did with FJ to write and record. She also didn't perform with them, of course.
Well, Kajiura is still evolving. As long as she keeps composing and staying inspired, there's more to come! I'm personally excited with the Aoi Teshima collaboration and her bringing in new singers.
Still waiting for her to write for Ensemble Planeta (she praises them through Hanae all the time) or other acappella or chorus groups.
Oh, and hopefully our main ladies can work out some projects with hiring Kajiura, if the terms and conditions are right, and they really want her paintbrush on their solo canvas.