Batgirl I could see, though probably played by someone older. One possible scenario is the film would end up with her getting shot (though by someone other than the Joker, of course) and then come back in movie 4 as Oracle - or spun off.
Reading a couple of the reactions to this rumor, I'm starting to think Nolan should abandon any plans to do a third Batman film. Clearly after the utter perfection

of TDK, anything that follows is going to be condemned. They shouldn't even bother.
I'm not saying that to troll. I honestly feel that in the wake of petitions by fans calling for the Joker to be retired from ever being used in movies again, and the OTT fawning over Ledger's performance (the most explicit case of James Dean Syndrome we've seen since James Dean; it was a good performance, I'm not denying that, but the beatifying of it went well beyond the pale of what would have happened had he not died. We'll see it happen again with the Terry Gilliam movie, too.) ... I don't think it is possible for the next Batman movie to live up to TDK. I bet a lot of people aren't planning to see it because they only bothered with TDK because of Ledger.
Better to fold when you're a winner, IMO. But the studio wants to make more money, so they'll be learning the lesson the hard way.
Cheers!
Alex