This is the idea for a little glyph. Just a cheeky little glyph that allows your shadowform to turn you into a K’thir much like the tidesages in BFA. You go do away with the ghostly effect that absolutely shatters transmogs with a different trade off. See K’thir use the undead rigging meaning they can show all clothing barring the helm. To me this would more closely embody the cultist vibe the Shadow Priest often leans towards. As for the void form, I’ve heard several ideas. Either a larger form of old god servant or alternatively just add the shadow effect to the K’thir.
In summary Blizzard has the tools. The ability to put armor on them and it takes away our transmog busting shadow form to give us the tentacles we deserve! 
much love!
