I dont have any specific theories in mind, but a few vague ones i can remember learning about. Iirc ancient people thought it was something to do with the muscles of the tongue or throat, i think comparing that to modern theories of being exclusively a brain issue would be interesting. Another one i remember from my childhood, is that most people use one hemisphere of the brain for speech and language processing, and people who stutter just use the other side so its affects our speech differently. That one never made much sense to me as a kid. Later on in life however after learning more about brain anatomy, i heard about stuttering being linked to abnormalities in the corpus callosum that connects the two hemispheres, so maybe theres something there.