Setup: "The Dark Knight Returns" is a classic 80's Batman graphic novel, followed by a series of sequels, written by Frank Miller - master of the genre.
TDKR is also what took Batman away from the light works of the late 50's through 70's back to his origins as a psychologically disturbed vigilante. The Batman movies with Michael Keaton too their tone from TDKR, but then producer Joel Schumacher went back to the silly theme, almost killing the franchise.
The story starts in the future with a 60 year old Batman coming out of retirement as Gotham again sinks into crime and corruption. The government doesn't know if this is good or bad, and even he and Superman are at odds.
I bring this up because at Comi-Con there was a "Watchmen" panel which included director Zack Snyder (300, Watchmen, Dawn of the Dead). During this panel he commented that he would "love" to do a film adaptation of "The Dark Knight Returns".
Most of the media let this pass, but one reporter decided to persue it further, going to Frank Miller and asking him what he thought about it. His answer?
"Zack Snyder can do The Dark Knight any time he wants to"
Call that a major endorsement.
This would, of course, have to be produced outside the "Batman Begins"/"Dark Knight" story stream as it does take place after the current timeline.
Now...will Warner Brothers take on such a project when there is easily at least one more film in the "Batman Begins" sequence?
Personally I'd take a big chance and do it, followed by sequels with stories along the line of those in the "Batman Beyond" series: a retired and ailing Bruce Wayne takes on a young personal assistant, Terry McGinnis, who takes on the role of Batman at night.
The "Batman Beyond" sequels would follow the story line used for the excellent animated film "Batman Beyond: Return of the Joker" animated film (especially the directors cut!!)