If I were to stage a performance of Macbeth, I would definitely include the character of Banquo's ghost. I believe that the scene with this ghost is very important to the story, as it allows us to understand the depth of the guilt and torment that Macbeth faces.
I would most likely use the same actor that played Banquo. This would give continuity to the play and would allow the viewer to easily identify the character. In order to make the actor look like a ghost, I would probably use white makeup to make him look pale, as well as ominous lightning.