Let’s assume this occurs in a DC reality where The Joker is dead or incarcerated yet again. If it’s the latter, he has multiple life sentences. He is incarcerated in a facility very similar to a Marvel Raft or Vault. As a result Harley is completely free of his physical presence & drifts closer and closer romantically to the Mistress of Weeds. Dating ensues and months, if not years later, the two wed in a surprisingly tasteful ceremony.
The pair return from their honeymoon and begin a chaotic spree of crime, the likes of which Gotham has never experienced. We’d see the “counter-ecoterrorism” that Poison Ivy is known for exploding in sequence with Harley adding her own zany twists to them. I have no doubt there would be a similar uptick in the crimes Harley would commit whether or not her Puddin’ was in play.
There would even be creative felonious combo platters that the pair come up with to have fun or make a point of some type. This 2 woman crime wave would last for quite a while, how long is uncertain.
The pair would find themselves committing acts of occasional vigilantism, such as helping a would be victim of abuse. They’d probably spare a victim here and there that Ivy wants to punish for ecoterrorism when Harley finds recycle bins and a bag stuffed with paper in each victim’s home. Then the big ticket act of converting goodness would come.
Harley will witness the abduction of a young “purdy” homeless teen. Harley will call Pam telling her what’s going down; to which Ivy would say, “Trail em babydoll, I’ll be there ASAP.” Harley would do so giving Ivy the final address, the two would besiege the place, rescue any inside, & beat the crap out of the abductors. It would be an act that breaks them, soon enough they’d find themselves in a vicious cycle of good violence.
Make no mistake, they will never be Justice League material, but a Bat family or Batman Incorporated material for sure. The two would be a fearsome, nigh unstoppable force of good in Gotham after accepting their newly found goodness. I would see them as a force that goes almost too far on a good day & absurdly far on bad ones. In time; however, they might stabilize as “Regular” ladies in the Bat’s roster of sidekicks. Perhaps the pair would claim Lucy and a few orphans as their children to become a true as possible family. Harley’s true love for Ivy and turn for good would erase all positive thoughts of The Joker from her mind. Ivy’s marriage in turn would help redefine her internal definition of who is and who isn’t a bad person.