Imo, melee can combo weapons, mages should be able to combo spells.

E.g.

Cure I -> Cure II -> Cure III

Cure I can proc the free cure I and if you combo with cure II it gets increase potency and lower mp cost and if you combo that with cure III it gets reduced mana cost and increased range.

or

Fire I -> Fire II -> Fire III

When used after Fire I fire II becomes a single target spell with increased potency from Fire I and FireStarter Procs in the middle of that make the combo on Fire III cost nothing (as firestarter does) but incrase potency by 150% for comboing.