For reference we've done:
Diplomacraft: The North (Before the Doom)
Century of Blood (Pre-Targaryen Conquest)
The Dance of the Dragons (Dance of the Dragons)
After the False Spring (Pre-Robert's Rebellion)
The Death of Kings (Alt-Battle of the Trident)
Lord of the Crossing (Before the War of the Five Kings)
The Iron Throne (Alt-aftermath of Robert Baratheon's death)
Clash of Kings (War of the Five Kings)
The Lion and the Rose (Alt-Purple Wedding)
A Feast for Crows (A Feast for Crows/A Dance with Dragons)
So what hasn't been tried would be the Targaryen Conquest itself, Maegor the Cruel/Faith Militant, the Rogue Prince stuff, the Conquest of Dorne, the whole Blackyfre saga, the Duskendale period, a direct version of Robert's Rebellion, and... the Season 7-8 stuff? (*shudders*)
Not that any of those don't offer opportunities, but I do wonder if any future ASOIAF game would benefit from another stab at alternate history.
I would like to put forward the Blackfyre period, specifically the first Blackfyre rebellion which is probably one of the most interesting scenario left unplayed.
