Some people have mentioned having Nolan direct TDKR. While I love Nolan's work and believe that the Batman films he has done will remain the best Batman films out for a while, I have to say that "The Dark Knight Returns" just does not fit his style. People say that TDKR is gritty and realistic. I'll agree with the first part, but its just not all that realistic. Miller doesn't really go for realism, he goes mostly for symbolism and all that good stuff -- at least, in my opinion from what I've gathered from his work. I mean, Joker escaped on a flying baby that spewed toxins from its mouth. He gave mind-control lipstick to prostitutes. The style of TDKR is closer to Watchmen than it is to Nolan's Batman franchise, and that's not a bad thing at all.
As for Snyder, I'll wait til after Watchmen to see if I like him for TDKR. Watching the trailer, I was conflicted. While I largely enjoyed it, I also had a kind of sinking feeling that maybe he focused on the effects and sleekness of the film and ignored the symbolism and the focus on the reimagining of the superhero. Of course, until the film comes out I'm going to giving him the benefit of the doubt and go into the film with high expectations.
As for Frank Miller, it IS his own work. It'd be interesting to see him translate it to film.
No matter how TDKR is made, it needs to be clear to the audience that this is a different entity than TDKR.