One headcannon I really love with Jon/Jonah specifically is the hair. OG!Elias having long hair (because c’mom it fits the stoner stereotype) and Jonah cutting it cropped short once he takes over the body versus Jon, who starts his job at the institute so desperate to appear the competent and confident academic that he keeps his hair short-back-and-sides and lacking any kind of personality at all. Then it grows out during his coma and he keeps it long, a last ditch effort to keep a piece of himself human, and by the time the apocalypse has started he just… keeps it. Maybe because he can’t be bothered to cut it himself, or maybe because he feels it’s a kind of defiance against Jonah. You made me into this but you do not own me, it says. I am still myself. The humanity that you so laboriously tried to get me to shun is still here, and I wear it proudly.