My real choices have already been thrown out but just to switch it up a bit, how about a series all about Faceless Men assassinations and espionage??
Since they are an international organization, one season can take place in essos, another season in the summer isles, another season in yi ti, another season in Westeros. Or mix it up and have certain events taking place in one region, and other separate but ultimately connected events on a different continent .
Faceless men killing rulers, taking their faces and impersonating them, fanning the flames of civil wars. Pretty much a show that takes the best of Littlefinger, Varys, and Jacquen, without the bullshyt of Arya's training.