By not balancing it.
By making it do something which makes the mathmagicians from EJ pull their hair out, like they love to do with Deep Healing. All it takes is a mastery bonus which's "bonus" is problematic to quantify, by not actually affecting damage directly, or only in specific scenarios.
Now this is just listing ideas which I can think up right now - and most of those are probably unusable in at least either of PvP or PvE - but how about
:
Mastery - Swift Demise
Increases your runspeed by 20%, stacking with all other movement speed modifying effects, according to distance between you and your current target, provided that target is affected by your Blood Plague disease. Movement speed bonus increases the further away the target is. Each point of Mastery increases your speed by an additional 2,5%.
(
I'd probably use ~30y as the full-bonus distance, adding a sort-of-charge like quality to Outbreak. Should decrease linearily with distance down to melee range, where the bonus is 0.)
Mastery - The Undying
When you die, your corpse will continue the fight for up to 24 seconds. You fight as a ghoul, but you retain most of your own abilities and ratings. Your ghoul will continue to fight by your side during this time. Each point of Mastery increases the time you can keep up the fight by 3 seconds.
(
Hey, I loved being able to continue as a ghoul, didn't you? Disabled abilities should be everything with a cooldown above ~30s, IMO. Mostly locking out tanktimers, ghoul summons, Army, Simul, Outbreak, such stuff.)
Mastery - Plaguelord
Your melee attacks siphon life essence from the target, fueling your diseases with additional strength. Each hit increases their damage by a small amount, up to 32% of your total melee attack damage. However, as the foe's life decreases, so does the power you can siphon from it. Each point of mastery increases the extra damage added by up to 4%.
(
In other words, while per-tick damage can go up massively - with 12 Mastery it'd be 48% swing-damage per tick per disease once fully stacked, effectively granting you another melee attack every ~2,8s - this works exactly opposite to Shammy Deep Healing. The higher health a target has, the higher one can stack the bonus. Ofc, attacking the target tends to lower it's health...)
Stuff like this.
Now granted, above is either underpowered, overpowered, or will break PvP. But the point is, Mastery doesn't have to be a fancy reading +damagedone. That makes it redundant, may as well remove it and replace it with +damagedone, aka, Str.