Viking, already said multiple times before but could always be said again.
A friend and I were talking in game tonight, and this is the system I proposed...
Bring back the classic FF jobs as advanced jobs in the game. Advanced jobs would be available after level 50, and you would have to CHOOSE which advanced jobs you leveled.
For example... I finally hit rank 50 on Gladiator, and at that point I can either perfect my skills as a well-rounded Gladiator, or I'd have the option to become either a Berserker or a Paladin. You couldn't do both, so this would provide opportunity for endgame customization. Plus, I like the idea of graduating into a traditional FF-named job... it would be like Cecil's transition to Paladin on Mt. Ordeal, or the advanced job attainment in the very first Final Fantasy.
I was tossing this idea around as well, ie MRD advancing to either Viking (DD tank) or Dark Knight (pure DD). Also making leveling the advanced job as hard as it was before the patch and allow casual people to sit on their R50 (now fairly easily obtained) starter job so they can experience endgame content too. I like the irreversibility idea as well and even perhaps making only a few (or none) of the advanced jobs' skills usable by other classes.
I wouldn't even mind if these jobs didn't get a skill every two levels either, 5 specialized abilities would be enough considering all the abilities we have access too because of how the armoury system is designed. For example, the DRK would need to level up THM for the Absorb spell line and THF would need to level LNC for Collusion.
Could just be a pipe-dream for the PS3 launch, but I don't see another way for SE to switch over to classical FF jobs given how spread out their abilities are on the current classes.