A New "Border" around the Skill: Once you master a skill, a shiny new border appears around that skill! The border could be Gold or Silver, or maybe you could choose a color for your "Master Border".
A new section for Mastered Skill: You know how you have little icons at the bottom of your portfolio that show your skills, followers, and favorites? They could put another little icon like that that would show your mastered skills.
A new skill set once you master a skill: This means, that once you master a particular skill, new challenges for that skill appear! You will only be able to access these "Master Skills" once you master the regular skills.
Unlock Xtra Avatars: This sounds cool! Once you master a skill, you gain a new avatar! Maybe once you Master a skill, you get that badge as your avatar! Like you master the animator skill, you could make the animator badge your avatar!
Unlock a "Master" Forum: What? This sounds really crazy, and really cool! Once you master a skill, you unlock a "Master Forum" where you and other Masters can talk about stuff!
So guys, whats your favorite Idea? I like the one where you unlock extra avatars that sounds really cool!
Keep on Mastering Skills!