Great docuseries running now, even though not by Ken Burns, it almost could have been - the Last Dance that ESPN is running about Jordan and the Bulls of the 90's. It was a great time to be in Chicago. Not sure how well it plays in other parts of the country, but it brings back a lot of great memories of watching Bulls playoff games outside in beer gardens/outdoor bars in the city with groups of friends.
As far as potential subjects, researching into the history of the Vatican would be interesting, as well as the Masons, and also maybe the Muslim faith.