I am working on a Thundarr The Barbarian inspired post apocalyptic campaign setting. It will include magic spells and items inspired by the GCRB magic system. It will also include mutations. I am having some problems determining how mutations might be implemented. Magic is setup to be unstable and failed magic will be worse for the caster then the example negative spell effects in the CRB. I want mutations to be more reliable as to be on par with failing or getting threats on a regular skill roll. Each mutation should have an extremely specific result, similar to Vancian spells. At this point I am leaning towards treating each mutation as a Talent but tracked on a separate Talent sheet, and advance separately, from the regular Talents. The mutations will be easier to use then spells with a difficulty of -D from the equivalent spell, minimum D to represent better control and stability.

Example:
Mutation: Breath Weapon
Base Spell: Attack
Additional Effects: Deadly
Difficulty: D
Skills: Ranged
Tier: 1
Ranked: No
Strain: 2
Mutant is able to project a single specific damaging substance from his mouth on a target that is within short Range. The substance must be determined when the mutation is acquired and cannot be changed later. This attack mutation has a Crit of 2

However, it seems like there should be a balancing mechanism to prevent mutations from becoming the preferred special ability over magic. This is where my issue comes in. How to limit the appeal of mutations so that they are essentially equal to the open ended but less stable magic spells. Perhaps some kind of negative mutation where, after so many Mutation Talents, the mutant must take a passive negative mutation. Maybe the Tier of the Negative Mutation Talent is based on number of acquired mutations, the overall potency of mutant abilities (total tiers of mutations), or some other measure. Mutants wont be able to become spellcasters with a high level talent, probably 4 or 5.

Any thoughts on how to approach this mutation issue, especially balancing mutations vs spells, would be appreciated.

Thank you for your attention.

Well perhaps mutations are better? I mean if you just make taking them a choice at character creation, then the person who took magic did so for Character reasons.

Just spitballing here, but I would look at the android setting and do them by reskinning cyberware, possibly with an essence or spirit limitation on the number of them you could have without becoming an inhuman monster (ie npc). If they are more reliable, but a PC can only take a limited number that could be incentive to go with the more flexible, but dangerous, magic.

What if the body could only cope with a certain number of mutations before it fails catastrophically?

Or you could take a leafs out of the Cybernetic Rules of the Core book. Each mutation reduces your Strain Threshold by 2, if so then perhaps they don’t require quite so much strain to use. This way a mutant can still perform a roughly equal number of uses of their ability to a Mage, but the Mage has more strain to play with doing other things.

An idea: in many post apocalyptic settings and games mutations often come with counterparts: defects.

If you go the talent route, perhaps each mutation has a defect that triggers on threat or despair. In your Breath Weapon example perhaps the add a defect:

Mutation: Breath Weapon
Base Spell: Attack
Additional Effects: Deadly
Difficulty: D
Skills: Ranged
Tier: 1
Ranked: No
Strain: 2

Mutant is able to project a single specific damaging substance from his mouth on a target that is within short Range. The substance must be determined when the mutation is acquired and cannot be changed later. This attack mutation has a Crit of 2.

Defect: Triggered with 3 threat or a despair, during the breath weapon attack the user accidentally swallows some of the harmful substance, dealing the full damage generated by the attack to themselves.

Do you want mutations to be things players choose, or things you assign to them? If the latter, I would think the negatives don't need to be too severe. Maybe the above breath weapon gets Slow Firing 2. You give each player one or two mutations that have minor impositions, and maybe add more over time if they're exposed to something that causes more mutation.

If it's something you want them to choose, then they could stand to have somewhat more severe effects.

You could have the difficulty upgraded by 1 per mutation on all mutations. Then just look to the effects of spending threat or despair on spell checks for some ideas as to what can happen when someone rolls poorly when using a mutation.