you know when you smell something so particularly bad that it lingers in your nose/scent receptors for extended amounts of time?
---in my headcanon, scarecrow's fear toxin not only causes visual hallucinations, but also hallucinations affecting scent. like, yes seeing scary shit would be terrifying, but what would really plus it up is if it causes one to just imagine the scent of say, charred meat/burnt hair/decaying carcass or flesh/etc.
maybe even after one of his victims recover from being exposed to the fear toxin, they just can't shake the smell of whatever for days and days, maybe even weeks, months or years? that's the type of thing that would absolutely drive me insane (and i think that jonathan would really enjoy that)
"Can i bring my pet OD'd Scarecrow to the villain meetup? He'll be so well behaved i prommy. Totally didn't put a brainwashing card on him" - Jervis tetch in this comic, apparently