In Killing Joke Barbara would be in her early-mid 20's and Bruce would maybe be around 30. Bit of an age difference but not ridiculous. Robins however are always teenagers, except for Dick after he grew up to be Nightwing. And seriously, I'm all for giving Barbara an actual character and some ass kicking and an actual life to lose. In the original comic (which was just a single deluxe comic… it's pretty short on its own) she's just a victim there to make Jim Gordon feel bad. Giving her any sort of agency and life in-story is a step up. Aside from all that, it's a complete stand-alone movie. A story on it's own. What does continuity matter? WHy is anything added to it "filler"? Make the stand alone story as strong as it can be. Long as its faithful to the spirit of the characters and still includes the whole story. (Even the original comic was meant to be an elseworld's sort of thing until everyone just embraced it and made Barbara's crippling an actual ongoing story element.)
It's not just the age, it's the character dynamic. Bruce is the mentor figure, the father, Tim and Barbara are his children. It's super icky that he's lusting for her. For crying out loud, she's batgirl not batwoman. It's doubly icky when you think that not only is he endangering Jim Gordon's daughter behind his back but banging her too. Also we'll see how much of this material actually strengthens her character and doesn't just reduce her to a character that does nothing but pine for a man…. This isn't encouraging. http://io9.gizmodo.com/the-killing-joke-movie-is-a-disaster-right-down-to-its-1784176711