In Marvel Comics, humans born with a mutated gene called the "X Gene" are called Mutants. Humans who undergo mutation of their natural genetic structure are called Mutates. In the DC Universe, beings with a "Metagene", a predisposition to gaining superhuman powers, are referred to as Metahumans. For many who grew up in the 90's, Mutants, specifically the X-Men, are emblematic of superhero culture.
Big Eyes, Small Mouth 3e
In many of the provided (Anime Multiverse) setting, Mutants fit in with little to know work needed. On Earth, Cathedral, Imago, and particularly Enid, the 'evolution' explanation from X-Men works perfectly fine. If a twist is needed, government (or alien) experimentation is not too far fetched here. Working Mutants into Ikaris is a bit trickier. Two possible reasons would be that of creating new 'half-breeds' of human and monster, or that the characters are the unexpected side results of living around magic-using characters, or in a magic-inundated environment. Regardless of the explanation or setting, the existing Attribute rules are more than sufficient to create any power desired.
Mage: The Ascension
The easiest way to fit Mutants into the Mage universe would be to simply give a character whatever spheres are necessary to duplicate the desired ability, then give the character a propensity to use certain 'rotes' that mimic the desired power. However, depending on how you wish to play these characters, this may be problematic, as there is essentially nothing differentiating these characters from other Mages. An alternate solution would be to explain them as a new class of Mage, somewhere between Hedge Mages and True Mages. A stressful event causes the Mage to exhibit a surge of Wild Magic, but they do not truly awaken. Instead, with practice and time, they find that they are able to duplicate the effects of the initial surge, albiet on a smaller scale. While True Mages understand that this is done through Sphere Manipulation, the 'mutant' is unaware of these semantics, and is completely unable to create any effects outside the manner of the initial surge. They are not in touch with their Avatars, though theoretically, they could be awakened to them. However, this would be fairly difficult, as the manner of the magic they use is extremely static.
Hackmaster
There is really no good way to introduce Mutants into Hackmaster. If it must be done, the Mutants will need to each be treated as new types of Monstrous Demihumans. This could be explained by a group of very powerful magic users (possibly connected to the CoSM) experimenting with augmenting humans to have innate abilities mimicking those of spells or monsters.
Children of the Sun
As I see it, it will be extremely difficult to integrate Mutants into CotS. As with Hackmaster, a possible explination is that of magical experimentation (perhaps by Lysirialite holdovers), but the rules of this system will not easily adapt to this type of ability customization.
Notes: While it is seamlessly simple to integrate Mutants into BESM as PCs or NPCs, it is not so simple in the other systems. Mage is plausible, so long as the campaign is designed for the unique needs of such PCs. For Hackmaster I would not recommend attempting to use Mutant PCs, and in CotS, I would not advise using the concept at all.
Notes: While it is seamlessly simple to integrate Mutants into BESM as PCs or NPCs, it is not so simple in the other systems. Mage is plausible, so long as the campaign is designed for the unique needs of such PCs. For Hackmaster I would not recommend attempting to use Mutant PCs, and in CotS, I would not advise using the concept at all.
