I need help thinking up the Jack of all Trades

I've got a general idea of what I want to do with my classes, but I am currently stuck on thinking about what to do with my Jack of All trades class.
Would it be preferable that I wait until I designed every classes before I think of it?
So far I've given him a bunch of low cost utility skills, paired with a speed buff passive the more you use skills. It's all good on that, but I want more of the class, but I don't knwo where I should go with it. Any help or general tips?

In my opinion, I would look to the Hero class in Dragon Quest games. They can fight, they can heal, they can use attack magic, and they often have a few spells no one else can use, like Zap or Kaclang. But keep in mind that their combat skills aren't quite on par with a Warrior, their healing isn't as good as a Priest, and their attack magic is weaker than the Mage. So basically, they can do everything, but not as well as a specialist in that field.

The thing with a Jack of All Trades (JAT) tends to be "Jack of All Trades, Master of None".

As @shockra mentioned, not quite as good in PHYS as Warrior.. not as great healing as Cleric.. Not as powerful magic as Wizard.. etc., so a JAT tends to be a very viable party member early on, but then mid-to-end game, their usefulness diminishes.

The trick is how do you scale a JAT party member to be moderately useful all throughout, rather than very useful to practically useless after being outclassed when other party members gain better skills.

Without knowing the intrinsic details about your battle flow, classes/balancing, etc., it's difficult to lend a hand.

One way I have balanced it a bit: my JAT class in my current project, is that they get the most benefit out of the "Amp'd Up" mechanic during battle. Basically, to make [Defend] more useful in battle, when a member uses [Defend], there is a chance of getting an "Amp'd Up" status. Amp'd Up gives different members different perks. When a Knight uses Amp'd Up, his ATK is doubled on his next Attack. When the Oracle uses Amp'd Up, her Status spells last an additional 2 turns.. so on and so on...

I've used this mechanic to empower my JAT member to remain useful through the whole game (as you cannot change party members once selected).

My JAT member is a Hunter, who uses bows, and specializes in dealing extra damage to beast/bird/etc. type enemies. His skills are your typical Bow Skills (Snipe, Rain of Arrows, Aim, etc.). However, his stats are just not as great as other members and the damage he deals is pretty soft compared to other PHYS classes. So what I've done with him, is when he [Defend]s, he has a higher chance of getting Amp'd Up than the other classes. When he uses his Amp'd Up booster, ALL of his skills change in some way. Rain Of Arrows no longer takes 2 turns to use.. Snipe no longer has a speed penalty.. Aim is automatically added to every Arrow Skill and Regular Attack, but if you use Aim itself as a skill, it increases the next normal attack to 10x the damage.. so on and so forth.

This keeps my JAT pretty much in the middle of all the other party member options, with lots of utility skills, but also keeps the Hunter relevant all through out by keeping him flexible.

This is all probably way too much information.. and I'm not suggesting you do the same.. I'm just offering a look into how I've dealt with the JAT in my project. :)

Hope you find it helpful!
-MIA

You give them skills that relate to functions from other classes, but make them more basic, or less powerful than from specialised classes. Or make skills that have unique secondary effects, like a protection skills that also grants healing on top of it. Or a skill that hits an enemy, and then heals an ally at the same time. That way, they can do damage (but less than the attacker role), and also heal at the same time (but less than the healer role).
You could also give them some uniques that might not fit any of your other classes, like maybe they're the only one to steal gold from encounters if you don't have a dedicated thief.

Highly inspired by D&D, mine is a bard. This character gets access to song magic, which puts them on par with my other 10 classes.

Mine was the Strategist class, which can invoke strategies. These are things like +5% ATK in exchange for -5% DEF, or immune to instant KO spells but at the cost of -2.5% of all stats. In other words, they can invoke trade-offs if you think it is worth having a boost to one area at the cost of another. In addition I made it so that you can only have 1 in effect at once, and in case you changed your mind and wanted none you can invoked a cancel skill which revokes all of them in effect on the party.

First of all, which kind of Jack are we talking about?

A. The Collector. A person with numerous skills. While he doesn't make thrones, he knows how to put chairs together. A Red Mage is a classic example. Another example is Archer, as not only is he a close and far-range fighter, but he can also access weapons with different abilities as opposed to a regular Archer that mainly does far-range. And classic Batman has a solution for everything.

B. The Jury-Rigger. A person who seeks a quick answer. While he doesn't know how to make a chair, he can make something to serve as a chair. For example, Kazuma only has enough points for an Adventurer and a useless Goddess, though the Adventurer class lets him learn skills from other classes; Kazuma was able to combine low-tier spells to escape from a high-rank monster. Goblin Slayer only spec'd one level into Warrior and Ranger at least, though he developed many ways to kill goblins; High Elf Archer gave him a list of things he can't kill goblins with and he agrees since his solution was not on that list.

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Point is, if your Jack is a Collector, good. If your Jack is a Jury-Rigger, then add different utilities from the skills he has to make up for the power difference. Also, one or two skills that he's good at or helps support the learning of new skills.

What kind of Jack are we talking about? The hero type, who is often a "jack of all trades, master of everything" and that's fine since they're the hero. However, if you're going for a more red mage type of character, be careful. It's very easy to make someone who is overpowered because they can do all the things (again, fine if it's meant to be the hero) and thus, are more character slot efficient. On the other hand, it's also easy to make this kind of character underpowered--I mean why take the red mage if the white mage is a better healer, the black mage is a better damage dealer, and the warriors all deal better damage?

I guess what I'm getting at is, if going the "jack of all trades, master of none" route, try to weave in some abilities that make him/her unique somehow. Maybe in addition to subpar healing and lower-tier damage dealing, they can inflict stat debuffs, have access to a damage type fairly unique to them, are heal-over-time based, or have some other utility that they excel in, even though they're just average at everything else.

I think having a Jack that's reliable could be nice. They're similar to Hero-type that they are more balanced and can fit with a lot of combination. My meaning is that, the shining point of these Jacks is in when you are in a situation to find a person in a slot that other choices can bring problem, sticking to the safe bet on a Jack can be a way to go. Having their own quirk is a bonus in this case.

Another type that is good is probably one that has to do with synergy. A skill that can change attack order or something like a quick cast could be nice.

you're talking about creating a hybrid class so logically speaking it has to be weaker than the classes its made up of or those classes would be pointless and it has to be a considerable difference. strategically speaking hybrids give you options you might not have if you took just pure class characters along. a half-tank half-mage might survive something that kills your pure mage but deals less damage. It'd be tough to calculate out the utility of every aspect of your combat but you can figure the basic tanky skills go to the hybrid and the very tanky skills go strictly to the tank. same with healing abilities or damage dealing magic/skills.