Make it change the amount of damage diseases add to abilities like, well only scourge strike.
I.E. Each disease adds 12% of SS physical damage as shadow damage, with an extra point of mastery it would be 13% and another point, 14%, etc.
This would work best if they replaced the set extra damage on festering strike with a disease modifier.
This allows mastery to still relate to diseases, a special part of unholy, and be desirable at least on gear, if not gems/reforging