Note: The following is not my personal fancast, just one that seems to follow the criteria in which Whedon would recycle actors for projects.
David Boreneaz as Batman
Love him or hate him, Boreneaz has the right look for both Wayne and Batman. And as we've already seen in
Buffy the Vampire Slayer the man can brood for an entire episode, also dabbing into some charm that Wayne would have to put on in his billionaire get-go. If Whedon's going with the people he can trust, you can imagine that Boreneaz wouldn't be out of the running any time soon.
Anthony Stewart Head as Alfred
Head is easily one of the best things about
Buffy, and he might be the epitome of British wisdom. Could you find a better fit in the Whedonverse? I should definitely think not.
Tom Hiddleston as The Riddler
A fan of wordplay, how could Joss Whedon miss an opportunity to bring the Riddler back tot he big screen? As we've already seen Hiddleston play a villain in green, I'd love to see the actor be a bit more free and gleeful than the reserved and emotional Loki we saw in Marvel's films.
Nathan Fillion as Harvey Dent
If Whedon went with the Riddler as a villain, I would suspect that he wouldn't overload his movie with villains (as he rarely does), which leads me to assume that Fillion would not become Two-Face, but would get to be his charming, determined self as Gotham's hardest working DA.
Summer Glau as Batgirl
Call me crazy, but I don't see Whedon throwing Robin into a movie as much as Batgirl. And with Glau's athleticism, acting talent, gorgeous looks, and ability to take a punch, who else could play Barbara so effectively?
Ron Glass as Lucius Fox
Fox is never
that big a role in either the comics or the films, and it's a role that could easily be filled by Whedon-vet Ron Glass, who played Shepard Book in
Firefly. Fitting? I think yes.
Cobie Smulders as Catwoman
Whedon likes to push his actors in new ways, and I could see him giving the whip to Smulders. After portraying a stuck-up, hardboiled SHIELD agent in
The Avengers, I'd find it a real treat if the beauty suited up as Selina Kyle.
Colin Firth as James Gordon
The only one currently not attached to Whedon by name, I just think that Firth could bring a lot of interesting layers to Jim Gordon, keeping him more in line with his
Year One persona.