Barbarra watched as the two villains conversed. Her proposition had sparked their attention, but now it was time to see if they would accept. Either, they would agree and Harley would join Barbarra in an endless binge of the criminals in Gotham, or she would say "Not a chance" and Barbarra would just be more food for Harley's belly.
Eventually, the two turned around, and Ivy said "We accept.".

"Really?" Barbarra's face lit up with a bright smile. Not only was she not going to be eaten, but now her plan would actually happen. The criminal element in Gotham was bigger than any other city in the country. You wouldn't be able to walk home without getting mugged. Sure once Batman showed up less criminals were on the street, but still, that didn't stop the scum of Gotham from attempting to rise again.
Barbarra always thought that was ridiculous, sure once a guy is locked up he has to serve his time, but he'll probably wind up busting out of the prison anyway. Even worse was the fact that the mob had most of the judges of Gotham on their payroll, so no matter what the charge, their boys would always go free. Last but not least, the super villains, aka the coming and going of Arkham. Every time they'd go into the nut house, they would either escape or say they had been cured.
Now up until now, Barbarra had tolerated this since she was a crime fighter and believed in the law, but now she had this incredible hunger for living flesh, which actually turned out to be a blessing because now she could keep these people off the streets forever.

"Absolutely, you would fill up Harley easily, but the proposal of a city-wide buffet is too good to ignore." Poison Ivy said. From the moment she heard Batgirl's offer, she immediately thought it was a great idea. First off, she knew how hungry Harley could get and she couldn't always be the same person for Harley to feast on. She would like to, but even she knew that other things had to be done if Harley wanted to become the reigning crime boss.
More importantly though, Ivy wanted Batgirl to stick around. She still only wanted to be Harley's girl, but as for the only girl in bed, not so much. She never thought there would be another girl nearing Harley's size, and now that there was, she was not about to let this potential new toy slip through her fingers.
Ivy knew she could use her sexual moves to talk Barbarra into a romp in the sack, and she had a feeling that Harley wouldn't mind either.

"Besides, getting rid of competition sounds tasty." Harley licked her lips. When Fatgirl had first suggested the classic 'sueperhero, suepervillain team up' she was still going to gobble her up. But then Ivy pulled her aside, and pointed out the benefits of this arangement. Obviously, eating criminals would mean less competition and send an even bigger message that Harley was the big girl of Gotham, in more ways than one. More importantly though, this brought to light potential meals that Harley never even thought about before. Sure she knew she would eat someone if they stepped out of line, but now she caught sight of how fun this would be.

Ivy walked over and began to untie Batgirl.
"You won't regret this, and trust me when I say, there will be a lot more of us to be seen." Barbarra was liking being and fat more and more everyday.
"Oh I have a feeling we'll be seeing 'a lot' more of you." As Ivy untied Batgirl's legs, she took another look at her ass.
"Up here." Batgirl pointed to her face.

Once standing, Posion Ivy brought something else to the crime fighter's attention "Now if you're going to be working with us, I want to show you my latest work.". Ivy quickly displayed a vile of green fluid. "I made this special chemical after Harley inspired me. It's similar to Chemical V, in the since that it accelerates the production of fat cells in the body. I even injected it into some plants." she explained.
"Red hasn't ofrgotten her roots." Harley joked.
Little did Batgirl know that one of those plants was the very one Batman had just battled. It contained that very chemical, which Catwoman was now drenched with.