Would love to see more frequent benchmarking of Debian GNU/Linux, especially as Ubuntu diverges further from it, making for interesting comparison.
The stable/wheezy release is of course covered already. So I'd suggest a test system installed with testing/jessie and kept up-to-date with new features going in.
From time to time, specific tests could look at interesting stuff picked from unstable or non-free. Maybe APT pinning makes for an easy way to do that. With some aptitude tricks it is probably easy to undo this and return to a pristine testing/jessie system.
There is full flexibility of desktop environment in Debian, unlike some distros which focus on only one of them. Very easy setup of Xen/KVM allows for testing those vs. bare metal performance on the same installed system. All in all, a very versatile test system, which might only need to be installed once.