I was rather bummed to hear the news, thinking Smith was good for at least one more season, hoping he'd go around 6. That said, he's starting to get film roles now and he may think this is the way to go.
I think Ben Daniels would be good, Jack Davenport would be an interesting choice, Ben Miles might also if they want to go a little older with the role without going truly patrician. Richard Madden is now available, though it would be tough for him to remain spoiler-free if he were actually in the lead role.
Andrew Lincoln, Harry Lloyd, Charles Mesure and Damian Lewis would also be interesting choices, though I'm not particularly fond of that idea. James Nesbitt would be awesome, and completely unlikely.
I think Ben Daniels would be good, Jack Davenport would be an interesting choice, Ben Miles might also if they want to go a little older with the role without going truly patrician. Richard Madden is now available, though it would be tough for him to remain spoiler-free if he were actually in the lead role.
Andrew Lincoln, Harry Lloyd, Charles Mesure and Damian Lewis would also be interesting choices, though I'm not particularly fond of that idea. James Nesbitt would be awesome, and completely unlikely.