Adventure Player's Guide seems like a good option.
What I am roughly visualizing is as follows...
Chapter One (Age of Adventure!): This is more of an expansion on the setting itself, various villains, heroes, organizations, etc. Also I'd incorporate the storytelling themes, seeds, and such as well.
Chapter Two (Mechanics): New rules, Knacks of all stripes, those sapient gorillas. Modern era knacks included (see below).
Chapter Three (Time Marches On): This is the looming fates we know will come. Discussing the death of Crackshot, the corruption of Primoris and the downfall of the Aeon Society, World War 2, and the Telluric Dampening. How everyone is ushered off the stage, until the Galatea incident.
Chapter Four (21st Century Adventure!): Adventure re-imagined for the modern day.