Something that might work is something like
- a card that gives a x % damage bonus fo the next x attacks with a potency of or above certain threshhold (e.g. 350 potency). Great for classes that have some hard-hitting skills outside their 2min burst phase like SAMs Midare and combo finishers, NINs combo finisher, DNC Standard & SD if it coincides with them having to dump Esprit, RDM melee combo and to a lesser extend DRG
- a card with longer/ shorter duration but weaker/stronger damage buff for classes that have less spiky dps of very short spikes
- a card that lets them use a skill without having it consume the special resource e.g. free Xeno, Midare, Shinten, Saber Dance, Bhava, Misery (yes, a card could actually be nice on a WhM), melee combo
- a card that gives x % damage bonus for every party member in a x yard radius, preferably with dynamic scaling checking with every server tick, making it stronger if the raid is (and stays) properly stacked and encourages rerolls when mechanics forced you to spread

There are more options but those are some from the top of my head that could work with properly adjusted numbers.

Freecure and Enhanced Benefic alter the next cast and it's possible to cast Cure II with Freecure below 1k mana, so it seems entirely possible with how combat and skills are coded to design buffs that enable the use of skills without having the required resources.
Not as easy to balance as flat damage increases because the cards have to be fairly close together in terms of rDPS if played correctly but ASTs would need to pay more attention to which card goes to who and where players are in their rotation. Higher skill ceiling than holding onto cards for as long as possible and then slapping them on the classic 90s/ 2min burst windows because they're a bit more situational and class-dependant.
Could also make for some fun combinations if you're lucky and cards with different effects stack like giving the resource-free skills card to someone with a damage boost for skills above x potency right after. Nukes go BRRRR.