Make city growth aware of the time period, such that pre 1850, growth should be very slow (1/8). From 1850-1900 a little quicker (1/4), then a new pace 1900-1950 (1/2), and "normal" (full) growth only from 1950. Some of us like to play with horses in the 18th century, but don't like megacities

I don't know anything about scripting in OTTD, but I figure this should be fairly simple to implement.