Warning: spoilers ahead for Episode 2 of Gotham Season 4, called "The Fear Reaper."
Gotham brought back Scarecrow in Season 4 to deliver some of the biggest scares of the series to date, and fans who have been waiting for his big return have to be pretty happy. As one of the biggest bad guys in Batman stories, Gotham had to do right by Scarecrow. Going by the events of "The Fear Reaper," we now have to wonder if Scarecrow's activities in Gotham will lead to the arrival of the true Joker.
In "The Fear Reaper," the Scarecrow blasted the Arkham warden with his fear gas, telling him that he would be forced to face his greatest fear. As it turns out, the warden just so happens to fear clowns more than anything else, which a whole plot of people can probably relate to these days. (Thanks, Pennywise!) The warden began to hallucinate that all the people around him were terrifying murderous clowns, and he went bonkers in short order.
The "going bonkers" wasn't unique to the warden, as Scarecrow managed to create an army of crazies in Arkham to riot and attract the attention of the Gotham City Police Department. When Jim (and only Jim) arrived, he faced an asylum filled with madmen and -women. Jerome Valeska, a.k.a. the guy with the demonic laugh who is the closest thing Gotham has introduced to the Joker, didn't appear in the episode, but the combination of the violent madness of the people in Arkham and the clown imagery suggests that perhaps Scarecrow's fear gas may be what pushes Jerome over the edge into becoming the Joker.
Jerome has been pretty off-his-rocker from his very first appearance, in which Jim learned that he had murdered his own mother, and he hasn't exactly gotten saner and more restrained in the years since. Still, Jerome hasn't had the edge of utter madness that most other versions of the Joker have, and he hasn't started going around in clown paint. Jerome is going to need a push if he's going to become a villain worthy of the Joker legacy, and being heavily dosed by the fear gas could do the trick.
Admittedly, Scarecrow's current fear gas can be counteracted by water, which is a pretty big formula flaw that wouldn't exactly irreversibly break Jerome's mind into the Joker, but the failure of his fear gas will undoubtedly motivate Scarecrow to develop a new formula. If Scarecrow gets too experimental, he might create something that he can't control or cure, and that would be the right recipe to turn a bad guy into the bad guy of Batman legend.
Jerome going full Joker because of a fear gas could also go a long way to wipe away his history with Bruce Wayne and Jim Gordon. If he loses his mind thoroughly enough, he could lose his memories, or at least lose his grasp of what are memories and what are thoughts/hallucinations, which could be a nod to the Joker's story in The Killing Joke. The odds are that Jerome is returning in the same season that brought Scarecrow back into the mix. Could Season 4 be the season that turns Jerome into the legit Joker, with the help (inadvertent or not) of Scarecrow?