You could look into riched20.dll and riched32.dll as wine is in need of these in a bad way :)
I submitted a patch to enhance the functionality of richedit a while ago, although it wasn't accepted (probably because I hadn't finished it). I have abandoned it for the moment so feel free to continue with the work.
If richedit doesn't appeal to you, maybe you could implement a few Var* functions in oleaut32. These are particularly important to VB users, and can be tested very easily since the functions have no side effects.
Rob