Thanks for your interest in traits. At the moment, there is no specific plan to extend the existing support of traits, since we have a number of development/support tasks of higher priority. However, your interest is duly noted. Feel free to file an "Issue", where we can track the to-do item. (Alternatively, if you are interested in going into the Dafny source to add this feature, I can provide a bit of handholding.)
Rustan
Rustan