Obviously it wouldn't be easy to test on Linux, but it would be really interesting to see the results of fuzzing against Windows Defender. Seeing as it's the de facto AV packaged with Windows 8, and a lot of people use MSE on prior versions, imagine the attack surface area.
Then again, if Microsoft is competent in how they built their AV engine, it might be properly fuzzed already. With such a wide deployment, you'd certainly want to think so...