You can use a big guy, wrestler or something, as the body double.
Use prosthetics and make up, and CG and animatronics.
and then CG a actor's face over, one who is a very good actor.
(like they did with Gimly in Lord of the Rings)
Also, if the JLA movie is CG they should cast Darksied as the bad guy. He would be perfect and easier to do in CG than in real life.
Who ever they cast, whenever he is to apear in whatever movie,
Darksied needs to have a distinctive/unique, and very threatening and powerful voice. Someone who can get across that his words are just as dangerous as his actions.
Maybe a great voice actor. Someone who does voice overs for their living.