The more I think about it the more I'm convinced that Lee Pace is the absolute best choice for this.
Great actor, great look, great voice, right age, has worked with Gunn before, experienced with franchises, and also seems like he'd be up for having the role for 10 years.
I don't think there's a better choice out there, he just makes sense, I'm actually shocked there's not more people rallying behind him because he's absolutely perfect for a comic accurate, Bat-Family patriarch version of Bruce. He'd stand imposing against all the other Bat-Family members.
I can find cons for pretty much every other choice but I can't find a single one for him.