It really depends. If it is resource based, in can vary based on how much oil/coal/ore/etc. is left in the ground, traffic congestion (as your delivery trucks can sit stuck in traffic instead of delivering and receiving goods and money), and the demand/price of your goods on the global market.
Things like tourism depend on how many people are staying and spending money in your city.
Mega-towers depend on how many people and how happy your residents are.
There are many other small factors for different city specializations but those are the major ones as far as I know!