I think 12 Byzantine Rulers - The history of the Byzantine Empire (19 episodes) is what Mike Duncan cited as his inspiration for the History of Rome (191 episodes). I'd also recommend Norman Centuries - A Norman History Podcast by Lars Brownworth (20 episodes) as being in the same style.

* Norman Centuries - A Norman History Podcast by Lars Brownworth

* 12 Byzantine Rulers- The History of The Byzantine Empire

* The History of Rome

Maybe it's just the scope or ambition of the projects but things like a season Serial Podcast or the Byzantine/Norman podcasts I mentioned here are more like miniseries.