It's welcome news. Christopher Nolan's first two films are good but the only female character was something of a cipher. I honestly assumed adding a female villain, specifically Catwoman, was the most obvious choice for the third film since... well, since I saw the second film.
Harley Quinn also works fine for me, although I don't know how that jester costume would square with the dark and gritty Nolan-verse.
Don't really care for the alternatives as I've barely if ever heard of them, but hey, Ra's al-Ghul turned out alright so if they want to do his daughter they'd better hire an Irish actress on the basis of more identifiable accents for me on the big screen hooray (also Liam Neeson's daughter)...
I mean, sure, whatever, but I'd rather Harley or Catwoman.