I'm planning a similar mod of my own.
So far Ive got:
1) Timescale to 0.50
2) Quadruple build times (in some cases I will rationalise build times somewhat, so a tavern can be built in a turn, but a cathedral in 20)
3) Increase build and unit costs.
4) Leave income as it is, since you have four times the time to accumulate income.
Also, does anyone know if it is possible to add a culture yet? I'd like to extend the game to 1600-ish and add in the Reformation.
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