Razgriz-Specter
Member
Because she is never in anything.You already have Harley. Twice. Three times with InJustice. And what kind of Origin story would it be if Joker already has Harley. Talia has only been in one Batman game and BARELY at that. She's also a much more plot significant character than Harley. Lady Shiva would also fit in much better to the premise they're setting here with assassins/mercenaries. A three way fight with her and Batman and Deathstroke, for example, could be really cool.
People want Cassandra Cain in every Bat thread to ever exist. There's some weird idea that she would be way awesome because she's not as popular as Barbara. She wouldn't make timeline sense for this game either, but I'd still rather see her as someone new than Harley again.
Edit: Oh yeah, she was a vendor in DC Universe Online......