We already have a "post level cap" system of progression, it's called ilevel.

The thing class/job levels are actually used for is story progression. That week you spend after an expansion drops, requires you to gain exp to progress through the story.
Other than that, it just serves as a gating system for playing new classes/jobs at endgame.

So I don't see why a 'master level' style system should be such a bad idea.

From a story progression perspective, it would be identical. Master level 10 would basically be the same as level 110, but you still only 'see' level 100 as the max level.
This Master level could be player specific rather than class/job specific. This way, you might need to reach level 110 to finish the story, but you only need to reach level 100 to play a second job at endgame.