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You could possibly increase the compression significantly more (e.g. 16:1) and run an Atkinson-cycle; to do this, you'd give the intake valve longer duration, and advance it to the point that it stays open partway into the compression stroke. It's easier to do this on engines with variable cam timing, because the fuel economy improvements come at the expense of power, and variable timing would allow you to switch it on or off at will. This is one of the tricks Toyota and Hyundai's hybrids use to get near 60mpg, and the loss in power is made up for by adding an electric motor to add torque down low.
On that note, I wonder if you could retrofit a servo or somesuch that would allow you to modify the cam advance on the fly, so you could get the best of both worlds - Atkinson while cruising, and OTTO while accelerating. You wouldn't be able to run astronomical compression, but you'd still see gains.
Other than that, you could play with the heads and see if you can get a stratified charge, to allow for leaner operation. My Insight is able to lean out to over 24:1 AFR on the highway, giving it diesel-like efficiency.
Other than that, taller gearing will probably make more difference than anything else.
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