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.
Or...
Expanding on this idea.
How about a show called "The Iron Bank" and it's all about the group of mega wealthy investors that run the iron bank? According to the world of ice and fire lore book, they have secret councils with a chairman of sorts, and men and women are on the council (big for a potential show because women are major characters in the GoT and no one would want a sausage fest). Make the series about them, and their agendas.
People always ask "what happens when they don't pay the iron bank? or "why is Tywin shook of the Iron Bank?"
Create a series specifically about what the 1% of planetos do to the world leaders that fukk them over.
They are pretty much the illuminati of asoiaf, as the economy of most of the world powers depend on them, and they have magical face shifting assassins that they can employ to do their bidding, so the series can take place in every single region that matters.
Create a plot where there is some political infighting within the Iron Bank itself, maybe even a "who dunit?" murder mystery in the pilot, where the chairman of the Iron Bank is found brutally murdered in the rainy streets of Braavos. And to escalate the events, the Iron Bank itself was robbed successfully for the first time
And what results is a "war of the five kings" sort of scenario, but it's not to sit the Iron Throne, it's to become the defacto leader of the Iron Bank, thus a fight to become, in spirit, the king of the world.
I just thought way too much about this.