As you price filters, you'll find a significant increase in cost from 58mm to 67mm. And stepping up to 67mm will probably preclude the use of a hood. So consider whether the additional cost is justified for the one 67mm filter size lens you have.
Good point, but I plan to acquire another lena with a 67mm thread.
Also hoping to get a 6x6 system someday. I am guessing they have a few larger ring sizes.