With circulating rumors about Fett and Kenobi movies in the work from LF, I got thinking about casting for Old Ben...and Ben Affleck, imo, would be a prime choice.
Say what you will about BvS, his performance was still impressive. He was a grieving man, struggling with his isolation and loneliness just like Obi. He was weary of the state of the world around him and fought back with pent up anger.
Imagine that performance in a small scale story on Tatooine. He could pull off the grizzled, angry Obi-Wan who lays waste to his enemies, while also having that intense brooding and brokenness.