"Instead of giant blades, why not a giant funnel? At the neck of the funnel a relatively small turbine generator. The funnel is a static structure, could be made any size, even increased/decreased in size without shutting down the powerplant, and would not require as much maintenance. The Venturi effect would dominate airflow through the funnel, making airspeed very high through the turbine."

The venturi effect creates a partial vacuum.

"All you would need is a big inlet and a small outlet. Wire and fabric could make up the majority of the skin. Consider a tv tower can be made 2000 ft tall, and supported with simple guy wires. Place two, 2000-ft tall towers 10,000 feet apart. Total funnel inlet area 20 million sq.ft."

You are creating an enormous sail. Nothing vaguely similar to a TV tower would be strong enough. Forget about the wind and just consider the weight of your sail material. Now, if for this wall of the funnel you had hundreds of tv tower like structures, that might hold up the sail material, but then you are back to the forces created by the wind. Assuming you could somehow make the structure strong enough- maybe 500' tall not 2000'- not with material we have available.
Next is this going to have roof? If not, a large amount of your wind will be forced up.

I think a better approach, would to use mountains and put a roof on the mountains that spans between them. Or create mountain like structures in locations desired. Making a roof that spans 2000' which is exposed the forces of the wind, would be quite daunting- perhaps not as difficult as building the Golden State bridge, but of similar level of difficulty and costs.

But I assume you looking at something that is cheaper. If you used your idea on a smaller scale and used small and most commonly manufactured wind turbines and tried to increase the wind power by say 50%, you might be able to make this work. But to be economic the cost of funnel would need to be about 50% or less than cost of the wind turbine. And if you could increase the power of a 5 mph wind to 7.5 mph wind it could make big difference because it could increase the amount of time in which a wind generator could generate power. So say for 15 m diameter turbine, you might make a funnel which was 30 m in diameter. In addition since at least some wind turbines have a max wind speed your funnel could also be made so it reduce wind speed:
http://www.entegritywind.com/ew15specs.html