Hi Daniel,
Again, welcome to Wine :-)
Finding good d3d-related gsoc projects has become harder each year, as the code is quite mature and we're out of easy things to fix. A few things that come to my mind are:
*) Writing more tests. Especially ddraw.dll could use a lot more tests and fixes for the pre-DirectX7 interfaces that are part of this dll, but also the dx7 interfaces.
*) The d3dx9 dlls are mentioned in the gsoc wiki page. I don't know how many medium-size tests (read: not HLSL-compiler related) are still left.
*) Speaking of d3dx9: An implementation of the vsa.exe, psa.exe and fxc.exe command line tools offered by the DirectX SDK may be a nice thing. This doesn't really require detailed d3d or gl knowledge either.
*) Fixing a bug of your choice may be an option. The risk here is that you get stuck finding out what the problem is, or that you end up with a problem that is not 3d-related and/or hard to fix. (e.g. the steam overlay had nothing to do with d3d, it needed a new feature in gcc)
*) Further implement Direct3D9Ex. This mostly comes down to writing tests, but there may be infrastructure changes involved that require in-depth knowledge of the d3d code.
*) A very long shot: Improve ddraw overlay support, e.g. to get some video players running. This *will* require very good understanding of Wine's d3d code.
I haven't looked into the details, so the time needed to accomplish either goal may be way off.