The Killing Joke is one of the most (in)famous Batman stories of all time. And ever since DC has been creating direct-to-video animated adaptations of some of it's more popular stories, this story has been thrown about as an idea for something people would like to see.
Mind you, this is it a pretty serious comic with some rather heavy-hitting implications. This is a comic that really explores how dark and depraved the Joker can be at times but also how he's actually pretty similar to Batman. And with this sort of subject matter, it's hard to consider how they might bring this to life.
What helps to make this particular movie so exciting is how it has brought together the creative team of the classic Batman: The Animated Series cartoon. Thus we get Kevin Conroy as Batman, Tara Strong as Batgirl and of course Mark Hamill as The Joker himself.
Apart from the trailer above, you can also check out an extended featurette about the movie here: