I've been thinking about how this kind of idea could be extended to other skills too. A few suggestions:
Knowledge 4 is an excellent idea, and I like Razrah's ideas too, especially the topside sups - they'd become very highly prized materials, and it'd be good to see that apply to things other than boss mats and Aen gear.
- Homins who've mastered all three pierce melees - pike, spear and dagger - could gain access to a short-lived aura which, for example, doubles (or even triples) stam consumption but completely ignores the target's armour. Very short-lived, to avoid unbalancing. Other similar things could be available for other melee types.
- Masters of CC and dags get a dodge boost, as a result of spending so much of their lives doing it, frequently. Could also apply to other melees, possibly as a parry boost where appropriate.
- For each dig/craft/magic/melee master after the first, an extra pair of HP/sap/stam/focus max/regen attributes become available. Would require some careful balancing, given the number of possible craft masters versus, for example, the lower number of possible magic masters - perhaps not one for each craft master but one for each new craft type master (HA, MA, shield, whatever major types weaps are split into etc), and so on.
- I'm sure dig multi-masters would love some extra map flags. :P
Knowledge 4 is an excellent idea, and I like Razrah's ideas too, especially the topside sups - they'd become very highly prized materials, and it'd be good to see that apply to things other than boss mats and Aen gear.