http://moviepilot.com/posts/2015/07/20/might-harley-quinn-be-in-dc-s-the-killing-joke-animated-movie-3397672
I'd be okay with this. While Harley didn't even exist when The Killing Joke was written; she's become so much a part of the Bat Mythos and the Joker in particular, that it's hard not to imagine her around in some capacity in pivotal events.
And I think the animated version of The Dark Knight Returns showed us all how uncomfortable a direct treatment of a classic comic 20-30 years later can turn out to be. Many of its elements fell a bit flat in a post cold-war world where things that were intended as satire in 1986 played as dull rehash with a more sophisticated 21st century audience.
