I suggest experimenting with Final Fantasy 11 if you wanna mess with cross class skills in a FF MMO. It's really fun to explore for the first time and there's a lot of different jobs to try out. The system lets you equip a subjob and gain it's abilities up to half the level of your main job. Jobs can be swapped at the moogle house. Final Fantasy 5 had a fun job system if you two want a fun single player game to play.

As for FF14 Bozja, Eureka pyros/hydatos and blue mage are the closest thing we have left to class customization. Lost actions let player decide if they wanna double down on they're roles strengths or off role into tanking/healing/dpsing.

My favorite example is Esscence of the Beast on Melees in bozja. It gives a lot of defense and a life steal affect on every attack and it stacks per target hit. This allows melee players to act like high damage drain tanks when paired with lost assasnination for a free constant Lost Font of Power Effect, which increases damage dealt by 30% and critical hit rate by 40%

I used this setup to solo the top side boss at he entrance to Castrum Lacus Litore. One of my favorite memories of bozja.

Eureka has similar abilites in the logos actions but they're limited to the last two zones and only one can be equipped at a time.

Blue Mage can learn a wide variety of spells and upon learning Aetherial Mimicry, blue mages can choose what type of spells they want to empower. Mimic tank boosts both defenses and empowers defensive spells like chelonian gate. Mimic DPS boost crit and dhit while empowering certain dps spells like matra magic. Mimic Healing HEAVILY Boost the potency of several healing spells like strotram, angel snack, and exuviation.