This suggestion is meant to work within the current limitations of the game, so please don't pull it off topic.
Part of the restriction was only having 15 abilities per class + 5 per job, to leave room for cross class abilities and also room to grow for when they raise the level cap (that last part an assumption, why else leave 5 slots open?)
So to keep that restriction, my idea was to use chaining to increase utility/potency, remove 2 spells to free up for other spells, and to make Ancient Magic better, since ancient magic kind of a joke right now.
Remove the third "tier" elemental variant, so this would be Thundaga and Firaga. Then change the first two tiers to work the way that Stone/Aero do. So Thunder, Fire, and Blizzard would be single target, and Thundara, Fira, and Blizzara would be aoe.
They would also be able to chain in both directions. Meaning using the tier 1 and then it's corresponding tier 2 would cause the tier 2 to become a single target spell, and also the reverse, using a tier 2 spell and then chaining on to it, its tier 1 would make the tier 1 become an aoe.
The exact level of power that they would be would have to be toyed with. Maybe tier 1 to tier 2, the tier 2 would be stronger than the tier 1, and for the tier 2 to tier 1, they would be equal in damage due to mp costs.
Now for Ancient magic. Ancient magic would still be single target or AOE however it is now, and would now chain on to either tier 1 or tier 2. So for Flare, going tier 1 to Flare, Flare would now be a VERY potent single target spell, and going from tier 2 to Flare, Flare would have the same amount of initial damage that it would normally, but also add a burn effect to all enemies hit, dealing damage over time.
For Burst, going from tier 1 to Burst, would have the same amount or slightly more damage than it does now, but would also add shock, dealing damage over time, and possibly adding stun or paralysis to the enemy, and going from tier 2 to Burst, Burst would now become an AOE, with roughly the same amount of power as a non chained single target Burst.
Ice and Freeze would get a similar treatment.
Over all this, in a sense give THM/BLM for control over what spells they can use for combat across varying situations, and would also free up 2 abilities of the 15 (+ 5 for BLM) +5 cross class abilities (another +5 for THM) ability limit, that could be used for any