Harley was certainly possible. She's just a Goddamn Joker fangirl. There's no logical reason she ever needed to even meet him, let alone be his sidekick. No, I don't like Harley Quinn very much. Worst kind of character. Subordinate distaff counterpart? Check. Blatant fanservice? You bet. Annoying, overblown regional accent? Believe it. Assumption the audience will ignore the fact that she's a complete monster because she's cute? Oh, you know it. Incapable of actual long-term growth? I'll grant I've not read 90% of Harley Quinn's appearances, but I'm willing to go out on a limb and say DOUBLE-CHECK.
They've already selected their villains: Bane, Catwoman to an extent and possibly Talia if the Miranda Tate character turns out to be her and maybe even Ras himself, so any character speculation discussions you missed out on in this thread you should probably go back 30 to 50 pages or so.
God, I just realized how crowded this movie is going to likely be. It's like they never, ever learn.