A number of us are already working on this. In my last playtest I was shooting for 500 turns per area to allow for 'period' gaming, and got roughly 400 (tech speed-up not entirely accounted for). Now that I know where the speedup occurs I can tweak things to slow it down.
Essentially just about everything in the game is rooted in tech. From that springs all the other changes you want to make, so you really have to get tech right in order to make everything else hang together. What I discovered is that - roughly, it depends on how you mod the game in other ways - you need to double the tech points required in each era to master all the techs available. It doesn't matter how many techs you have or what you call them, although it will matter if you allow players to ignore a substantial part of the tech tree to race ahead on to the next era. So the ratio over six eras is 1/2/4/8/16/32.
If you're shooting for 500 turns/era and you don't change much else about the game (except for slowing down production and improvements somewhat, to avoid fast development), the tech points per era come out to about:
Ancient - 35,000
Classical - 70,000
Medieval - 140,000
Renaissance - 280,000
Industrial - 560,000
Modern - 1,120,000
The earlier eras may go slower if you limit ICS, or if you do something foolish like disallow idle cities to build gold (which they'll need if you don't want to go immediately broke from maintenance). If you want just 200 turns for each era, you can multiply the above totals by .4 and use the results, and so on.
The point totals themselves don't matter; they'll change depending on what else you do with the game. But the ratio of 1/2/4/8/16/32 seems to be a valid one for making each era last a roughly equal amount of time. The game seems to be designed to speed up along this scale.
Max