Suicide Squad: Kill The Justice League - 10 Things It Must Do
7. Don't Make Heroes Out Of The Villains
One of the biggest appeals of playing as a villain is the ability to not be a good person, so why try to make those villains out to be heroes?
These characters being villains does not mean that they cannot have relatable elements or even certain lines that they do not cross, but as they are villains, let them be villains. People still fell in love with the MCU's Loki even in spite of his two attempts at actual genocide, for example.
In regards to Harley Quinn in particular, if the game connects to the Arkham series rather than being in the rumoured new shared game continuity, then having her suddenly become a hero would make her more or less a different character to the version seen in Asylum, City, and Knight.
People can change, sure, but not every comic book protagonist needs to be a hero.