IIRC, Miller auditioned for Superman when Brett Ratner was involved and that led to him playing the lead in Prison Break, which Ratner exec-produced. But I don't really see him as Batman. Maybe Dominic Purcell, his PB, co-star, when younger.
Whoever plays Batman in this film needs to look like he could hold his own with Henry Cavill. Yes, obviously Batman can never match Supes for physical strength but if you have a small, scrawny Batman standing beside Cavill's imposing hunk, the disparity is going to be so glaring.