Easiest and most sensible route, imo, is Ling Qi's domain weapon. It's currently some kind of flute-dagger thing(it's been morphing shape some), but it was crafted from a chunk of solid darkness we fished out of Huisheng's pool, which he's vaguely referred to as his 'blood'. He managed to involuntarily yank it out of Ling Qi's dantian too, once.
Point is, our domain weapon is linked to Huisheng/is reflective of our initial (inadvertent) introduction to him. And Huisheng has horns, while also being representative of a number of Ling Qi's meditations on the history of the Emerald Seas, while also being a teacher contributing to the tools and philosophies carrying Ling Qi forward into the future she wishes to build.
Having the domain weapon's form change to a pair of solid shadow Weilu horns on her head would be fun and cute. It's not like she really stabs anyone with it these days; it's for empowering mists, etc.. It'd also be something that isn't out/obvious most of the time, only being revealed intentionally or when she's in full-serious fight mode. That element of at least temporary secrecy makes it all the more fun to play around with in the narrative, imo.
Edit: we also have additional huisheng gunk in a head meridian already, and I could easily see him pulling a "prank" in terms of nudging our domain weapon's continued evolution, shape-wise. There's a lot of material to work with there, basically.