D&D uses slots. Mana and slots are ultimately the same: unlimited power with no consequences until you run out of gas, then no power with no consequences. Meanwhile, fighters have limited power all the time.

Hard to balance. D&D's failure to balance this in all but the most ridiculous of scenarios (i.e. X encounters per long rest) has undermined the balance of the game for decades.

So what would be better? Unlimited power with unlimited gas?

Or perhaps...no gas, but some consequences all the time?

Posit:

• Every supernatural power has a power level (let's use S6 rank, 1-5)
• The possible consequences scale linearly with power level
• Every manifestation requires a skill check. Always.
  • Perfect success: you get exactly what you want, consequence free. Yay.
  • Mixed success: you either get what you want with an equal consequence, or pull your punch and nullify the whole thing.
  • Failure: you don't get what you want, but there's still a consequence.
• Possibly, the concept of "Mana" or "combo points" could involve allowing casters to upgrade their result by adding consequences.
• Consequences can include:
  • Blowback: self-harm, such as HP damage, ability strain, debuffs
  • Collateral damage: harms the intended target and some non-intended ones too
  • Misfire: doesn't exactly do what was intended, but still mostly works (e.g. polymorph enemy into sheep? Nah, you polymorphed him into an angry bull)
  • Hex: everything's fine, but your next spell is fucked (or maybe the DM can play the Hex whenever, keeping you on your toes).
  • Raid Monster of the Week for more ideas.

Ah, but what about rituals? Epic spells?

You mean the bane of my existence? Maybe just fuck 'em for now until they learn to behave themselves.

Power Tricks

What if you want to do something outside the specific bounds of your powers? Well, you can...maybe. Here are some guidelines:

• You cannot increase the magnitude of a power. So you can't weave Fireball and Force Push together to do double damage. You could, if you wish, weave them into a fiery force push that does normal damage but combines elements of both.
• If you are combining two powers, you are doing two things, and thus must pass two checks, the lower of which determines success. Three powers, three things, etc.
  • If you voluntarily downrank your powers (say, you are using two rank 2 powers even though you cast at rank 3) you can negate one level of this disadvantage.
• It need not be the same caster weaving powers together; multiple casters, sharing an action, can interweave their powers to make interesting combos. Everyone always has to make at least one check, but you can use "help another" logic where all but the highest roller are rolling to "help out" the main caster, which gives favorable results compared to everyone rolling separately.