There are characters in World of Warcraft and League of Legends who can gain stacking passive buffs when using certain offensive skills, or when taking damage from specific attack types, and then consume these stacks of buff when conditions are met to do something.
For example every Ice Blast spell might give you a stack of the “Icy Veins” buff and when you get three, the fourth Ice Blast spell will consume all “Icy Veins” stacks to do 1.5x damage, but your Blizzard spell will instead consume those stacks for 1.2x more damage. A Fire user could gain 1 stack of Heat for every tick of DOT burn damage you inflict on the foe and spend those Heat stacks on making his fast fire spells stronger or reducing the time his slower stronger fire spells take to cast.
A lightning user could gain 6 stacks of Static per fight he’s in capped at 40, lose one Static stack at the end of every turn, gain 1 Speed for every 10 stacks of Static he has, and his strongest move can consume all stacks and deal extra damage for every Static stack it consumes.
A system like this could work in Fire Emblem with Skills and Combat Arts/Spells. There are already temporary stat buffs and debuffs. A future game could expand on the concept. A Mage’s most powerful attack could give the user six stacks of Burnout, preventing the mage from attacking until the stack has depleted by one every turn, and by another stack each turn the user spends not moving. An Armour Knight could start out with 60 stacks of Undented Armour granting him increased block chance and damage reduction only to lose some stacks and some block chance and damage reduction with each attack he takes. An Assassin’s basic attacks could inflict stacking debuffs on the foe that reduce the foe’s armour or accuracy or base stats temporarily, or use Combat Arts like Sunder to damage the foe’s defenses more and Poison Strike to debuff the foe and deal poison damage that grows more severe with each Poison Strike taken. Staff users could buff allies with stacks of a buff that decline in power as their stacks fade with time. Healers might gain Light when healing allies, and spend that light guaranteeing a Miracle proc or dealing huge damage to enemies.
A system where characters manage and spend a limited resource they generate through some class-dependent means seems like a way to add more player choice to the moment to moment decision making of Fire Emblem, and balance Combat Arts to require more resource management in the fight.