Why Harley Quinn?

Because it's about time. She is one of the most interesting and loved villains in the Batman universe but somehow she is yet to make it to the big screen. Her insanity could be both relatable realistic and unpredictable, done well, she could be the coolest Batman villain in cinema. She doesn't have to be the main villain, even though it would be awesome, but either way we have been waiting for her long enough.

"Man of steel" included some round interesting female character. Sadly, it's still a fact worth mentioning. You could go one more step and present us with a strong female antagonist who for once won't be someone's love interest [because you won't do the mistake of using the joker again so soon right?]. We, the audience, are ready.

Why Jennifer Garner?

Because she is a good actress that looks the part, a proved action star and is able to be adorable while childish. She could do a relatable three dimensional Harley as well as a fun one, and won't fall into the trap of being just loud and annoying.
Please?