Wild concept for a Jason Todd villain that ISN'T the joker
Ironically though, based on another evil clown.
Violator from Spawn takes a very similar niche to someone like Joker to Batman but the entire set up is different; Spawn is ALREADY a bad person that lived an entire life of killing and perpetuating a cycle of violence. (Not that I'm implying Jason is bad in this comparison, obviously) - Violator's job is to corrupt him even MORE and snuff out what light and love he has left for the world (Which is largely tied to his still remaining family)
Something like that would make an interesting dynamic for Jason Todd/Red Hood where a villain specifically targeting HIM is trying to do the same. Jason already kills people but his motivations are entirely to control crime and stop the real monsters from doing what they do to innocents. Having someone constantly foil or screw with him by constructing elaborate plots that make him make hard or illusory decisions or constantly put innocents in the crossfire of him and his targets (kinda like Violator does) would be a fitting issue for him - he's already straddled the line of villainy before but even at his worst (except maybe Gotham War idk) he doesn't try to hurt people that doesn't deserve it, the existence of that ethical line is why he isn't just a full on villain to begin with. Joker is like this to Batman to an extent, but with the Violator it isn't just "I'm gonna make you break your code" it's "I'm gonna make you totally forget your entire past life and make you a totally walking void of anger and murder".
The problem with this working, though is that unlike spawn, or really most superheroes, he doesn't have a large cast of people he cares about that can really be fucked with to be grounds for emotional manipulation. There's the batfamily, which DC is already super iffy on who can do what and who can die and such, and... not much else, even ignoring the obvious issue that if this villain tries to go after Nightwing to screw with Jason they'd be bringing Batman down on them too if Nightwing doesn't just kick their ass.
It'd also run the risk of just being too similar to Batman and Joker's dynamic, and "how much worse can they make jason get" already kinda gets answered in shit like gotham war and arkham knight to an extent but I dunno I feel like there's something there. If they want him to be anything other than Batman's used cowl that had one or two really good stories post resurrection they need to have him explored through more personally intimate villains that aren't just batman hand me downs. But knowing how obstructive DC writing politics are, I doubt it lol