When it comes to the film canon of Glenn Ficarra and John Requa, two titles jump out that would make a perfect mash-up for this Joker and Harley Quinn movie: Crazy Stupid Love and I Love You, Phillip Morris.
Though it should also be noted that Deadline has labeled this project flat out as the upcoming Gotham City Sirens project, which would seem to hint that Harley and The Joker will be the main focus of the film.
What this means for the other two Sirens, Catwoman and Poison Ivy, is to be left to fate.
So with those two films standing out in the Ficarra/Requa canon, it would seem that this project is hypothetically going into good hands.