I can certainly see the potential there (and it is not like we have not seen a thousand hacks play with such systems for similar reasons) but how you would avoid it being overpowered whilst also making people want to play it vs having specialists, or everybody having similar powers by one means or another, I don't know.
That's just an example from FFT, there would be a different set of things you could manipulate in other games.
To balance power levels (lol): power should be directly correlated with particular or conditional limits on utility.
Bear in mind that I like the ideas behind the Final Fantasy job classes, I just think they're poorly implemented. My ideas are alternative implementations. And to get to Blue mage I kind of need to cover all the other color mages, so please bear with my effort post. I'll use FFT as an example.
Feel free to skip if you're impatient.
Example 1: Black mage should IMO get access to most elements except holy and oddball effects (gravity, poison), and should have several levels of each spell with varying power, MP costs, and if applicable casting speeds. Maybe range and AoE too. Since all of its skills are powered by MA, and they have long charge times, it needs MA most of all and isn't served by particularly high SP. Let's call it a Power Mage.
Logic: The Black mage as elementalist blaster lets you hit every weakness, which if your elemental affinities on enemies/gear are balanced should always give you a good offensive option. This also gives you an option for synergistic builds around elemental absorbing gear/classes/characters/etc, which gives your Black mage two roles: primary elemental damage dealer, secondary elemental healer.
Example 2: White mage is almost perfect, it just needs slightly improved buffs and holy elemental damage spells that work just like the Black mage. As its skills are more useful if they are put on your party before they close with the enemy, and many of its skills don't use MA that much (buff hit rates are MA + X, while healing/damage amount is MA * X), it can afford to give up some MA for SP. Call it a Speed Mage.
Example 3: Green mage (AKA Oracle/Mystic) needs better debuffs, and the AoE and hit rate of its skills should be improved. It's two damaging spells - Drain and Osmose - use MA for hit rate IIRC, or not at all. I think their damage should be MA * X instead of a (X * max HP / 100), so they are more useful for other mages. Either way all its skills are (MA + X) hit rate, and getting two turns to inflict a debuff is more useful than 1 or 2 more percentage points to inflict a debuff, so it should be a Speed Mage.
Example 4: Gray mage (AKA Time mage) is similiar to the White mage, but is more offensive support while White mage is defensive support. Its only damage options should be 2 or 3 Demi spells, and a very powerful and slow Meteor spell (which I want to be the only non-elemental damage spell in the game). I really want the Demi spells to do (MA * X) as a percentage of the unit's current HP, and to accept an element (Dark). While its role as support mage and its fluff seem like it should be a Speed Mage, I think it should be a Power Mage since I also think it should promote from Black mage and because I want all types of stat growths available at any level.
Example 5: Summoner should IMO be the generalist Power Mage: it's slow, it's powerful, it has a wide AoE, and it can do everything that all the other mages can do. That said, it should do so in a limited manner: massive MP costs, randomized (1...n) de/buff infliction/cancellation. Since it's slow and it's damage spells are (MA * X), it should be a Power Mage.
With all that in mind, and the "monsters must be able to use these skills too" consideration, here are my ideas for a FFT Blue Mage.
Stat Growths: Balanced PA, MA, and SP growths/multipliers. It's spells don't necessarily rely on MA, and it can use something to do when it hasn't learned many spells, so it should have decent PA and gear options so it can stab things until it grows a bit.
Gear Selection: Probably everything but heavy armors for the head and body slots. It could stand to have knives, swords, rods, staves, sticks, and crossbows.
Skills: Have a way to do everything that can be done in FFT, but in a way unique to you.
* White Wind: Heal up to 5 targets in a cross shaped AoE up to your current amount of HP.
* Explode: Kill yourself and damage everything but you in the AoE with your (max HP - current HP).
* Bad Breath: inflict 1 debuff at random from the list of all debuffs, AoE of 3.
* Reincarnate: turn a monster into another random monster; or switch a generic human's gender flag, change their job to a random one that they have available, randomize their equipped secondary skillset and skills from those available, and randomize their zodiac sign. There will be limits on this depending on the target, since only a certain number of sprites are available on each battlefield.
* Seagull's Snatch: Steal a random equipped item from the target.
* Sunder: Break a random equipped item on a target.
* Sacrifice: Kill yourself and revive all your allies on the battlefield at 1 HP.
* Forest Song: Remove a random debuff from all allies in a wide AoE.
* Dragon Soul: Add a random buff to all targets in a 5 square, cross shaped AoE.
* Soul Steal: Absorb a % of current HP and MP from a target adjacent to you.
* Transfer: Give a certain % of your current MP to target within 3 range.
* Blood Transfer: Damage your HP by a certain %, give half that amount in MP to a target within 3 range.
* Steal Essence: Reduce an adjacent target's PA, MA, or SP by 2, increase your own by that amount.
* Heart Feed: Reduce a target's Brave or Faith by 20%, increase your own by that amount.
* Warp Time: Damage yourself by a certain percentage of your HP, damage CT all targets in AoE by that same amount (10% HP damage = -10 CT).
* Blood Sin: Damage a target in 4 squares by the amount of your current Experience, and reduce your Experience to 0.
* Renewal: All allies in an AoE have the duration of their debuffs halved, and that of their buffs doubled.
* Curse: Single target has the duration of its debuffs doubled, and that of its buffs halved.
* Potion Spray: give all allies in a 5 range line a the lowest available potion.
Some of these are terrible, some are unbalanced (but that's easy to fix, just tweak the AoE, range, hit rate, damage, and so on), but the gist of the Blue Mage is that it is the wildcard generalist: it can do anything that any other class can do, just not as good as them.