I've been looking around for a good project for a group of ten students for the spring term. In 2006, I had them work on implementing missing features in richedit. Most of the easy features there have been implemented now, I think, so I looked at msxml3. That's not a great candidate, as a fair number of the problems with it would require knowledge of COM to fix.
So at James Hawkin's suggestion, I looked at comctl32. Sure enough, only half the controls seem to have conformance tests, and there's a fair bit of documentation and tools to help newcomers.
So I updated http://wiki.winehq.org/Comctl32 a bit, and I'll update it a bit more as time goes on with suggested projects for the students. If you're interested in common controls issues, please visit that page and add any corrections or project ideas that come to mind.
Thanks, Dan