Not a headcanon but it’s in the list, but the thing I think about the most in relation to queer identity in Ensemble Stars is Sora’s bisexuality.
I wonder if it has to do with the way he sees colors? Maybe he’s gender blind as well? Either way it’s on my mind a lot. I love you Sora. It’s pretty much canon so… that’s why I picked it there? He fell for Euryale as a child despite their lack of gender ++ Medusa as an adult. It’s nice to rotate in my brain. He also describes Medusa’s beauty in a similar way that he talks about Shishou, lol.

Umm, other than that, HiMERU I think as genderfluid more out of the role he takes, as he must be able to switch everything about him in a second, and it intertwined itself into his gender identity.
Umm.. Ibara within the fandom actually gets headcanoned as such very frequently due to the bogie time story and mostly card art, (I headcanon her as a lesbian because of bogie time) however, I have my own reasons, rather about the way that Ibara has grown up creating a point of view on a ‘masculine role’ in society, and how Ibara does not necessarily fit into it.

Those are only a few examples, if you want a specific about my thoughts on something, I could answer! It’s a bit difficult to bring certain thoughts to mind however.