I think Douglas Monroe will appear in probably the second half of Season 5. Who should play him? Here are two people who I think would be great in this role, but it probably won't happen unfortunately.
1.) Bryan Cranston - If they were able to get Bryan Cranston on The Walking Dead, he would be awesome at the role of Douglas. He looks exactly like him and since he is such an amazing actor, they could keep him around for possibly 5 seasons if they could and really expand on the role. However, I don't think he would do it. Although he probably would be interested in the role, he would cost too much money because for Breaking Bad he was paid almost like a hundred thousand dollars per episode. Unfortunately, it probably won't happen sadly. But that would be incredible if it happened.
2.) Jonathan Banks - He played Mike on Breaking Bad and he does look alot like Douglas. He's such a good actor and if he or Bryan Cranston played Douglas Monroe, I would literally shit myself. However, Community already announced that Season 6 is confirmed and I think he'll be involved in that and plus, the Breaking Bad spinoff, Better Call Saul, he will be a series regular on that show and he has too many things going on, I don't think he'd be able to. And plus, he would cost alot of money because he was paid alot for Breaking Bad as well. It's a shame because Jonathan Banks is a terrific actor and would be awesome at the role.