Given just how far away Harley Quinn has gone from being the Joker's DCEU sidekick since Suicide Squad, the franchise has the perfect opportunity to replace her with his new comic sidekick, Punchline. Given David Ayer's original vision for the Harley/Joker dynamic was never realized in the final theatrical cut (despite positive fan reaction when those plans came to light) and the return of Jared Leto's Joker to Justice League, the motive and opportunity are both there.
Punchline entered into the world of DC comics in April 2020, and for all intents and purposes served as the new Harley Quinn; an aid to the Joker who shared his penchant for clown makeup and for trying to defeat the Batman. While not everyone was excited to see this character - partially as those who thought her first entry into the world would make them a quick buck bought up every comic she was featured in fast - she has served a very clever purpose. Joker hasn't really had a proper sidekick since Harley stopped being at his beck and call, and the Clown Prince of Crime often needs another major character by his side to balance everything out a little, even if this is just to let him monologue to someone the audience recognizes.
After Birds Of Prey, Margot Robbie's Harley Quinn currently has a slightly precarious future in the DCEU, and Punchline could help fix this. Harley is caught between two iterations of herself - the version seen in Suicide Squad who appears to still have some affection for her clown counterpart, and the version seen in Birds of Prey, who had clearly broken up with the Joker and wanted nothing to do with him. It wasn't clear whether she'd continue on this trail of learning to stand on her own two feet like in the current comics, or if she'd once more return into her abusive relationship with DC's biggest villain in order to justify giving the Joker more screen time. Adding Punchline to the mix could offer a resolution, assuming Harley Quinn sticks around and the Joker returns to the DCEU. Crucially, those who wanted to see a devilish clown duo together could be satisfied, and Harley could still remain free to continue with film endeavors that no longer involve Joker, like a sequel to Birds of Prey or her own solo outing.
Such a decision would actually have another benefit as it would potentially give Punchline a little more character, too, as due to her fairly recent creation in the DC comics, she doesn't have the same kind of detail behind her that many other DC characters can boast. Given that Harley herself became a DC staple due to her appearance on Batman: The Animated Series, allowing her sidekick successor a similar chance on screen to become as beloved by fans seems pleasingly fitting.
Most importantly, the introduction of Punchline would prevent one of the biggest repeated pitfalls of Harley's DC comics' history, when it comes to the Joker. Instead of Harley going back to the Joker every time various writers wanted to put the two together for a scene, she would be given freer rein and allowed to develop further independently of her former partner-in-crime. And as long as both characters remain active - or at least possibly active - in the DCEU, there's potential for them to be drawn back together, which would be a disservice to Harley. The DCEU had a rocky start, including how Suicide Squad dealt with Harley Quinn and Joker's relationship, but seeing future films fix one of the biggest issues related to them - and not just as a cathartic cleanse as in Bird of Prey - would feel like the franchise was fully aware of its failings. Introducing Punchline could offer that ready-made solution.