After seeing some posts recently about Blue Mage, I saw something which gave me an idea; someone compared how it could work to the cross-class ability system.
The abilities that Blue Mage can learn could work like cross-class abilities, but with a twist. Blue Mages would get both preset abilities, as well as ability "slots" as they level up. Through encounters with enemies (or potentially players or pets in PvP), Blue Mages can learn abilities they can slot into these "ability slots" which they'd unlock as they level along with the preset abilities. These unlockable abilities might not necessarily completely match how they work when used by enemies, for the sake of balance, but it would still be apparent what they were learnt from.
However, the ability slots would have a catch: each ability slot would have a particular function tied to it, and only abilities with matching functions could be slotted in each. This way, it's not possible for players to create broken builds (in one way or another), but still allows players to learn and use a whole swath of abilities as the Blue Mage is known for its ability to do. Potentially a player could unlock every single ability possible to learn as a Blue Mage, but they would only be able to use as many as they can slot, and only as many match the functions for each slot.
This kind of setup also is somewhat similar to what I've heard might be coming to allow Summoners the ability to have additional Egis; a "pet glamour" system. Same ability "slots", but a wide variety of unlockables with different looks and feels to them, even if they share functionality.