Another thought we are toying with is coming up with a penalty on subsidy if mining with gpu as to keep the playing field level.
In Other words gpus would be nerfed down to around acore I7 reward rate.
I'm not sure how you could possibly implement that. Who would want to intentionally nerf their GPU miner, and others will just change the source code to do that. (and of course, no one will just mine on pre-compiled binaries...
Or rather, why would I mine with a GPU when I will mine the same using my CPU. The whole point of the GPU (and FPGA and ASICs in the distant future) is to mine more.