OK, I will be detailed. Recall that this is a spitball idea that took less than 10 minutes to think up.
Before listing spells, I will go ahead and point out that they already have a system to limit you to 24 abilities. Assuming some percent of abilities ARE broken, just make sure the 24 at time of queue aren't, a boolean value on the spell saying it is not allowed. OK, spell ideas that are easily tweakable (took list from wiki and customized to easily balancable effects):
Generic moves from many games:
Angel Whisper - Standard resurrection spell
Mighty Guard - low duration shield to compensate for aoe (shake it off)
White Wind - like convert, but instead of giving mana, gives health to a target
Frog Song - Inflicts Toad on an enemy, boss immune/dungeon restricted
Magic Hammer - Does damage and returns MP (same as energy drain).
Night - Aoe Sleep. Functions identically to BLM.
Aero - inflicts Wind elemental damage single target. magic damage, wind marker on BLU gauge
Aqua Breath - inflicts Water elemental damage aoe cone target. magic damage, water marker on BLU gauge if it hits
Bad Breath - Bad Breath inflicts various statuses in cone. bosses would be immune to all but the dot portion.
Flamethrower - inflicts Fire elemental damage single target. magic damage, fire marker on BLU gauge
Doom - Doom inflicts the Doom status on one enemy. boss immune/dungeon restricted
Level 5 Death - Level 5 Death inflicts Instant Death on all enemies whose levels are a multiple of 5. boss immune/dungeon restricted
Roulette - randomly KOs an enemy or a party member. boss immune/dungeon restricted
1000 Needles - 1000 damage to an enemy, ignores defense, trait up later.
Acid - Acid inflicts non-elemental damage to an enemy, applies vulnerability
Blaster - inflicts non-elemental damage and attempts to inflict Instant Death on the target. boss immune/dungeon restricted
Goblin Punch - inflicts non-elemental damage to one enemy, low damage, fast cast time.
Lifebreak - potency increases the lower BLU's health. single target, long CD
Limit Glove - high damage move that can only be used by consuming all 6 elements on the BLU gauge.
Matra Magic - non-elemental aoe high damage, long cd
Self-Destruct - sacrifice for huge damage. boss immune/dungeon restricted
To balance out the gauge mentioned above, the following would also be needed (all 7 gauge moves required for dungeons - would be core gimmick)
Earth Shake - Circle aoe of Earth damage. magic damage, earth marker on BLU gauge if it hits
Electrocute - Circle aoe of Lightning damage. magic damage, lightning marker on BLU gauge if it hits
Cold Wave - damage over time Ice damage. magic damage, ice marker on BLU gauge
You could actually take even more of their elemental abilities (11 added a ridiculous number), so that they have aoe AND single target for both, and have more than one option to consume the gauge.