I really like the way Cities XL did the industry.
Heavy Industry > Manufacturing > Hightech
Each higher class polluted less and hired more educated workers. They also feed into each other. Using stuff from this game we could say that Ore, Oil, and Forestry supplies the basic zonable Industry. That basic industry then supplies input materials to the Manufacturing Industry which would be handled through districts, like the Ore and Forestry ones. The Manufacturing Industry would then supply input materials to the High Tech Industry which again would be handled through districts. The High Tech industry would supply input materials to the Commercial zones.
We could take this a step further and make different Commercial Districts. Restaurant, which would be supplied by the Agriculture Industry, Residential Retail(think home improvement stores, malls, and car dealers, gas stations) which could be supplied by all the industries including High Tech. The largest buyer of High Tech Industry though, would be Offices. They would buy lots of computers, printers, and other High Tech stuff. Now all of these Industries would buy offices in return. Think of it as a corporate headquarters for the large industries. The large retail conglomerates would need to buy offices for the same reasons as well.
Now all of these things could be imported from the outside world but businesses would make more profit(and more taxable revenue) if they could source them locally.
Sorry I went on a bit of a tangent but I think this could really add some needed depth to the ingame economy and the game overall.
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