One basic decision would be, if the effect of negative happiness is nationwide or city specific. In CivV happiness is a per-nation value, not a per-city value as in former versions. Therefore I would prefer only nation-wide effects of negative happiness / dark ages. If we want city-specific effects like city becomes a city-state or some of the pop turns into rebel units, we also need city-specific happiness (displayed in each city), so that the player knows upfront which cities are in danger. That would be a big change for CivV. I think we had that in former Civ-Versions so we are used to it and it seems natural to us, but in CivV Firaxis had decided that happiness is nation-wide, I think we should give that idea a chance. I propose to reduce the population (-1 pop in each city, -2 in annexed without courthouse) in the first dark age, maybe more in subsequent plus -75% production for all but happiness-buildings + puppet cities concentrate on happiness-buildings.