I wonder how big the filter threads would need to be?
Depends on your definition of need. With long lenses you have a hard lower limit on the filter size, not so with the wide ones. Here you want the front element to be as bulbous as possible to avoid a shallow angle of incidence.
Compare the Nikkor 17-35/2.8,16-35/4, Canon EF 16-35/2.8I and 17-40/4, as counterpart we have the Tokina 16-28/2,8 which has got really good corners.