Aside from Hugo Weaving (V) I cannot think of any other actor who was humble enough to stay behind the mask if required. So I do not expect this to suddenly change. Flashbacks could be a good excuse though. McGregor is an experienced actor; a solid choice IMO. A bit surprising nonetheless though; after some of the more weirdo Black Canary reasoning I'd rather had expected Tony Todd to play him (which if I'm honest I would quicker to buy in a terrifying antagonist role).
edit:
Just read he voiced Darkseid in some Lego DC Movies. Maybe they get him for the big bad