Final Fantasy 5's Job system
Final Fantasy is a varied series, always changing how it's played. A few installments feature a job system, giving the player a wide variety of ways to customise and approach fights. Change to a knight to become a durable physical attacker, a black mage for magical offense, or white mage for healing, as well as many other more interesting options.
In Final Fantasy 5's case, this system is considered so robust it can be completed with any combonation of 4 jobs. This spawned an annual event - the Final Fantasy 5 4 Job Fiesta (hosted at https://www.fourjobfiesta.com/).

But even so, I'd say some of these jobs could stand to be improved, as well as the player's ability to customise. I mean, it was released in 1992, after all. So I'm going to look at what they made and put forward some ideas for improvement, hopefully without changing anything too massive.
The core systems
Before I get to the jobs themselves, I think we should look at the core way the job system is mangaged. Overall, I'd say it's pretty good. The player is more or less introduced to the jobs in four blocks - about 4-6 jobs at certain story events. Doing this instead of drip feeding jobs throughout the story lends itself to the FF54JF system quite well, as it gives four distinct points where the player is given their job for this run. But even without this player enforced challenge, I think it's nice to be shown a bunch of tools at once for the player to start chewing on. It could be argued that this might be a bit overwhelming, but I would say that the individual jobs are simple and well constructed enough to not cause this to be a problem.
More customisation - Looking at another FF game with a job system, they allow for "slots" that can be added. FFT has a 2nd ability slot, but also a reaction and passive slot. FF Dimensions, if I remember right, lets you get more slots to equip abilities when you progress in a job - but only while in that job.
big n small abilities: One solution is maybe spliting the active abilities into... I'll call them big and small abilities. Some abilities are pretty strong and robust, like the spell lists for black and white mages. But sometimes a job doesn't come with, but instead levels up to learn a small, not so robust ability. Which is harder to justify using, compared to these strong abilities.
I don't think the reaction slot would be anything for this. Unless a bunch are added, it just wouldn't work. But having a distinct passive slot definitely would help, because as it stand there's rarely a case to equip them outside of a couple instances.
Problem Jobs
Some jobs are better than others. To be fair, in a single player game and having so many to choose from, this is going to be inenvitable. But some really struggle, even considering this.
In particular, I want to focus on Red Mage, Berserker, Thief, and Dragoon.
Red mage: Everyone who's played it know that having only level 3 white and black magic doesn't do anything not too long after you get them. The abilities simply don't scale well enough. But giving them too much access to both school would be overpowered, so it's a careful balancing act. If nothing else, I think they should get level 4. This would give them a couple more buffs, a status heal, a better damage spell and some instant death-ish capability with Break.