Tech trees shapes:
Fall From Heaven has a "thready" tech tree. In it, you are encouraged to go deep down one particular branch of the tech tree, because there is very little cross-links, the cost of tech doesn't grow fast, and the later techs are more efficient than the earlier techs.
Civ4 has a "base" tech tree. There are some cross-connects, and the rate of tech cost goes up at a moderate rate.
An "Era" tech tree would have massively increasing costs and possibly serious cross-connect in the tree. This would encourage you to finish off an era.
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To make an "Era" tech tree work, you'd want each Era to end up massively boosting the research rate of the civilizations.
Ie, if each Era had technologies that boosted your commerce production by 50%, your science by 50%, and your city sizes by 50%, the expected research growth would be on the order of 3.4x, and your production would grow by 50%.
It would be worse than this, because you could afford a larger empire with those higher ROIs.
Or the Era could boost your science production by +400%, and +50% commerce. Then your research budget would be boosted to 7.5 times each era.
Done carefully, you would have civilizations reaching the end of an era, having a massive research boost, cleaning up the past era technologies, and moving on to the next.