While watching season 1 as it was airing a friend bright up the two time periods theory and that the the Man in Black and William are the same.
I said I didn't think it was true because It would remove the narrative anchor they had built the show around. There wouldn't be any thing to keep audiences emotionally invested in the plots that are going on.
I loved the 1st two seasons! Although season 1 was far better. Season 3 I have vague memories of and season 4 I gave up. They never should have left the park, that was the best bit.