Especially considering that female characters who are overweight by default (examples include characters from Total Drama, Fat Princess, etc.) are often a source of disgust by mainstream audiences due to them considering fat to be ugly.
That said, Harley herself has been shown to be two-sided over the years since her first appearance. Sometimes she can be evil (like in Batman: The Animated Series and the Injustice games), and sometimes she can be good (like in the live-action Suicide Squad film and her original psychiatrist persona before falling in love with The Joker). It’s the same way with other characters that tend to have both a good side and an evil side.
The scenario ideas are actually good, if she were to properly retire with Poison Ivy, another female Batman villain that she loves almost as much as The Joker.