Instead of "limited," make it an "elite" job. Instead of just levelling up and being able to faceplant into all new content whether you know how to play or not, you have to actually work for it. It still can't go into anything it can't currently go into, until you unlock a specific set of abilities, or even all abilities, and pass some trial. After you have done so, you are allowed to use it in normal play.
Basically, It remains almost entirely the same as it is now, but people who actually get their abilities and pass that carnival whatever can use it in roulettes and such. All they'd honestly need to do is adjust cooldowns, costs, etc on skills so that a maxed out blu isn't a flat out god. Then, keep adding ability/competency checks every 10 levels before allowing it into the same content as everyone else at that level. Anyone who can't cut it can't play it, anyone who wants to play it better learn to do it right and pass their test.