My idea for job mastery is meant to target and highlight the subjob class system in FFXI. By "skillfully" utilizing both your main and sub jobs, access to special abilities are rewarded to those who have mastered the usage of both of their jobs.
Each job and subjob will have it's own unique ability or weaponskill based on how the subjob supports the main job. Whm/nin would have it's own exclusive abilities completly different from rdm/nin.
Blm/Sch Job Mastery Example
My blm/sch has landed the killing blow on 20 mobs while under the effect of Alacrity. This "skillful" play allows me to use the exclusive blm/sch job mastery ability, "Multi-cast."
Multi-cast allows my blm to select and use 3 spells at the same time. Per the example, it is only through usage of my sch in a certain manner that allowed my Blm to meet the prerequisites and use the ability, and thus is the essence of "skillful" play.
Melee ideas:
Example:
2 friends are duoing in campaign battle and are experienced in job mastery. They have earned the job mastery weaponskills exclusive to their jobs of nin/dnc and mnk/nin. As masters of their martial arts, it's not a surprise that their martial art spirits would connect and allow them to perform a unison weaponskill after they have met the requirement of achieving 5 skillchains together. (Taru nin leaps off the galka's shoulder to land a fatal blow? Yes, please). Anywho, for melee, i'm thinking a special weaponskill is awarded for job mastery. A step further would be the mentioned team weaponskills that interact with players in the same party/alliance/whatever.
For meleeing, you have their own unique weaponskills and abilities they could unlock, but I think adding combo weaponskills could be another cool feature of Job Mastery.
Similar to magian trials, a number of prerequisites must be achieved SOLO to call yourself a true master of the chosen combination of main and sub job. I say solo because the idea behind Job Mastery is that you achieve a personal mastery of your job. Running around with a group quickly completing the trials defeats that purpose.
I want to say the prerequisites for achieving each Job mastery abilities would be set high, but is something that COULD be found in simple exp parties. When a player kills a mob doing a certain amount of dmg while enhanced by mambo, a brd receives credit for that kill. This may include a group but it's something the bard is not forced to do. He is utilizing his current abilities to their potential.