In a way, "Advanced Classes" and "FF Tactic" style advanced job gains are already in the game.
Judging from what some wikis appear to have in terms of abilities learned a PLD can be composed of
Main: Gladiator (Sword + Shield) (lv 50)
Maurader:Defender, Intimidation, Axemanship (lv 20)
Archer: Shadowbind (LV 6)
Pugilist: Taunt, Second Wind, Prime Condtioning *increase heal rate on self by 10%, Season veteran(lv 20)
Conjurer:Cure, Protect, Shell, Stoneskin, Radiance, Repose, Spirit Bind,
Trance Chant, Fast Cast, trait that gives +10 to conjury affinity (Lv 50)
Thauamturge: Firm Conviction (increase def while casting), (Lv. 20)
While some abilities really aren't defined as PLD it does help to allow the PLD like class to have more support
and freedom over how effective their curative spells can be. With every class have a different % of magic in the
fact that all classes have some form of MP, it's possible to create PLD through a mix of classes as if one gained
the class through an FF tactics like system.
In terms of an advanced class, you could say the advanced classes come from the aspect of mixing and matching abilities
to created a new play style for the class of your choice. I would say that if a PLD wished to stay through to the "light"
side of PLD they might want to remove some abilities listed but if you wanted a PLD that took advantage of keeping the mobs
attention and having effective ways of binding them so they can't run away as well as traits that allowed you to increase your
defense, you could use the above as a base and then look through more combinations for creating the class.
If one is concerned about their spells not being as effective something to keep in mind that I've heard about is while it appears a person can wear any gear they wish, each gear type seems to be designated for Fighting, Mages, Crafters, Gatherers, so if one wished to be a PLD they could cover themselves in heavy armor for the DEF and HP boost while putting on mage like gear where the defense from those slots isn't as high as from the ones that matter to gain more MP and other attributes to improve healing.
It'll definitely be interesting to see the variety of "PLD" setups made by the player based on their own personal playstyle but going back to the OP, it just comes down to how we interpret the abilities we gain to create the advance class of our choice.