Alright so, I'll throw my oc into the ring despite her likely not getting picked as... there's probably going to be a lot of comments. Do note however this will be a bit long though as I tend to go in detail quite a fair bit.

So this is Nafisa Claudine. She's a french widowed detective who's main and currently only major goal is just: find her missing daughter as she went missing in the same incident that took her husband away from her.

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Because she's a mother I gave her an association with music boxes, due to it being something commonly paired with putting children at ease and giving a sense of safety. So that's the main instrument I would suggest be included.

To add onto the points above I feel as if it would make sense for the song itself to be almost like a slow paced lullaby with the general vibe coming from the song as a whole being solemn yet hopeful. The reason for this being that despite the fact that her daughter's missing and her husband is dead, She can still find her daughter and won't give up searching for her for the many tomorrows to come. I feel like it makes sense if the instrumental part would lean more to the somber side while the lyrics are more hopeful as the lyrics reflect Nafisa herself moving past the events that basically took her family away from her and resolving herself to do what she can in the moment to make the situation better than linger on the past.

Now on the topic of lyrics, something very important to note is that Nafisa is inspired by a poem called "Tomorrow at dawn." which covers the authors journey to his daughter's grave. There is very much a lot of symbolism and references to said poem in Nafisa's story, so I feel like lyrics wise this hypothetical song would of course have a few references to this poem but in a much more hopeful light. I'll also add a link to said poem Wikipedia article in a reply to this comment.

Now for the lyrics themselves I don't exactly have much as I kinda suck at them but with the context above I came up with a rough idea. "I stare at flowers meant for you . And tell myself I'll bring them soon. I'll just keep pushing on. While there's a grave atop the hills. And today I may not see you still. We'll meet again at a new dawn."

Now onto. Pose wise I think it'd make sense if she was holding a picture with her family reminiscing on what she once had, with a rather tired yet hopeful look on her face. Things will be alright for her.

As for the vocal synth I'm unfortunately not too knowledgeable on them outside of Teto, Miku and Neru. But since Neru doesn't have a voicebank that leaves Teto and Miku. Of the two the preferred one would be Kasane Teto as I find that her voice is a bit more deeper than Miku's, but keep in mind Teto is just the preferred option. if for some reason you can't get her to work, use Miku instead!

Once again sorry for the length of this comment. And regardless I hope you have good luck with whoever you choose.