I know that ages are hardcoded into the game so that new ones can't be added, but some new settings could be added that provide interesting "intermediate" ages. These settings would be maximum age options that would enable certain units an age early.

Post-Enlightenment Age
When the world was just barely entering the Industrial Age. Riflemen, machine guns, destroyers, pre-dreadnoughts, monitors, submarines, and artillery become available.

Post-Industrial Age
The world's interesting technological state just before and during the outbreak of WWII. Fighters, bombers, aircraft carriers, and armored scout cars become available.

Post-Modern Age
Characterized by the Cold War and the various conflicts around the globe. Assault infantry, jet fighters, strategic bombers, jet fighter-bombers, attack helicopters, anti-air missiles, armored cavalry, attack submarines, and cruise missiles become available.