4. Get Rid Of Robin & Batgirl
There's no polite way to say this, Robin and Batgirl are lame and unnecessary characters who add nothing to the franchise and their portrayals by Chris O'Donnell and Alicia Silverstone sucked. Batman has always worked best as a lone figure against the world and the father/son, mentor/student thing with Robin has never worked well. For a 5th flick, the characters could be written out as having branched out to another city to spread the crime fighting legacy further afield, or if WB were feeling really daring they could kill them off in the first act to up the stakes and give the story a take no prisoners, anything goes kind of feel.