Fresh classes.

[Kitru]

An Onion Knight style class that gets stronger based on your number of level capped jobs would be interesting. No idea how that would fit into the Holy Trinity, though.

There's already something like this in game insofar as you can run as a base class and use 10 CC skills drawn from all of your classes instead of a job with 5 drawn from 2.

However, if you're talking about a class that actually grows in power based upon your number of maxed out jobs/classes, it would be broken as hell. It would need to be balanced around having all jobs maxed out because, otherwise, if you maxed out all of the jobs, it would end up being stronger than any other job. At that point, it then becomes weaker than any other job until such time as you have maxed out all jobs.

Onion Knight/freelancer works in other FF games because they're single-player games where balance across classes isn't really needed because you're not competing against anyone else. Freelancer starting terrible and becoming the greatest fighting force in the universe is simply a reward for completion in a game where completion isn't really required. It's kind of like the ultimate weapons found at the bottom of the uber-dungeon after the uber-boss: you've already beaten the only thing that said weapon would actually help against, so it's only really there as a "you are this kewl; have a badass weapon to show off and demonstrate your awesomeness" feature.

The same logic is there for elemental weaknesses in MMOs as well as a bunch of other stuff. MMOs absolutely *have* to be balanced amongst the classes so as to not render one or more classes irrelevant for running content and play has to be both fun and approachable so a lot of the classic tactical/mechanical options that other games used are outright impossible as well.

Honestly, when considering mechanics, it works better to think about ARR as an MMO, rather than a Final Fantasy game. The theme/aesthetic can be derived from FF, but the mechanics most definitely need to be MMO.