Golden Ages and Dark Ages
Ok revisiting the era ideas ...
Using the Ages from Civ4 RoM and then adding the much needed "Prehistoric Age" at the beginning and "Galactic Age" at the end. Note that the "Galactic Age" is more or less Civ Fuehrer's "accent of Mankind" project.
Anyways this takes the idea of the "Golden Ages" and "Dark Ages" a step further. If you get a "Golden Age" you unlock even more techs for the age your in. For instance if your in the Renaissance you would unlock "Da Vinci Age" themed techs like this "cog tanks" or "Ornithopter" as a pre-flight aircraft. Likewise if you get a golden age during the Industrial Age you get "Steam Punk" themed techs like "Steam Robots".
On the flip side if you hit a "Dark Age" you go to the dark age of that era and not only can loose techs from that era but not be able to research some techs normally available.
Also if you get a 2nd "Dark Age" during a "Dark Age" you will go back to a previous era Dark Age. And if you have a 2nd "Golden Age" during a "Golden Age" you get to go the the next era's "Golden Age". Once you finish a "Dark Age" you will go back to your current era. And if you finish your "Golden Age" you will advanced to the next Era.
Note that some techs will not be research-able once you have past an era. So it is important to research all of the techs in your era before advancing to the next era. Not doing so could result in not being able to have the prerequisites for the next era's techs.