Alright if you want a idea I was thinking we could start from 4 points.
A. Being before Aegon the conquer and his dragons with the 7 kingdoms.
B. Being Aegon's begining of his conquest where you play as one of the kings or a command should we get enough.
C. Being Robert's Rebellion. You could alter history Maybe robert dies and stannis takes hold and is crowned king maybe the rebllion is crushed who knows,
and finally D. War of the 5 kings. Take hold and lead your kingdom to glory or should we get enough play as a lord/lady of the land and bring political intrigue into this.

Once we get what time period we want we could go over other rules.