Regarding haircuts, have you thought about splitting up the "haircut" section similarly to how expressions are split up?
Like, instead of having a bunch of prebuilt hairstyles, you could just have us manually pick from different menus what bangs, hair length, etc. we wanted?
From there, you could also include options for extras, like different ahoges and so on, that could function like accessories, but for hair.
From looking at how the files are split up as SVGs, I think this should be possible - at the very least, when customizing characters myself, I'm able to switch this stuff out with relative ease myself.
Another thing I'd personally like to see would be different coloring or shading styles, if possible? Working between the new nightly build and the old, old stable build, I can't help but feel there's something sharper and cleaner in at least how the older build handled hair.
Beyond that, I suppose a big one would be more eyebrow, eye (shape and pupil), and mouth variations. Right now I'm taking to customizing all this stuff myself now that I can dig into the SVG files, but having some more built in stuff would be nifty too.