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There are many improvements made on the 'throttle blade' opening over the years, from 'rolling cylinder with an oval hole' on this Aurora indy car engine (shown), to some single and double guillitine types, an iris type, and double opposing blade design I've seen. A friend's sponsor applied for a patent on one he's used/using on his 4 cylinder in-line Ford USAC midget. They are mostly useful in some types of race engines, ones where part-throttle acceleration and low end torque for exiting a turn are useful. But the only engines I've seen them on are one throttle per cylinder (no plenum type intake tracts). I am thinking that a plenum is busy enough, load of turbulence but the ports and intake runners 'straighten out' the air flow after the throttle on a plenum design.
Still, I think about this stuff too. What if you had a 'revolver' type throttle body design. Where, like a revolver pistol, the correct venturi size was quickly rotated into position for optimum venturi diameter for a given engine load?
If you think about GM's XFi, where they used a slightly smaller throttle body than a standard Metro, there must be something to be gained (I assume GM wouldn't have bothered if there weren't mpg gains to be had for specifically the XFi modle of Metro)
enough of my rambling...
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