It is said that Quirks are a part of physical ability. They are non-existent until at least age of 4, increase drastically during puberty, and wane in old age. They become stronger the more you use them (and the greater the load is) just like muscle.
Too much load can back fire on the person's body and cause injuries. This can also happen during "second awakenings" or Plus Ultra moments.
But they are also VERY different from muscles in the following:
>Unlike muscles, they can go through exponential growth in a very short amount of time, sometimes at the expense of the User.
>They do not seem to abide by the world's laws i.e Law of conversation of energy or Laws of Thermodynamics(even though there have been attempts to explain it, they seemed either futile or half assed).
>Attempts to justify conservation of energy seem unsuccessful as the scale of exchange of energy is too skewed(like in cases of Momo or Todoroki).
(its a Shonen anime, no need to beat yourself up on details that you can just gloss over)
Quirk Names: Were Kaminari, Tetsutetsu and others named after their Quirks? Seems unlikely as Bakugou was known as such before his Quirk even awakened(he was friends with Deku and he called him Kacchan)
Quirk Identification- Most likely someone with a Quirk that can recognise other Quirks names and registers them(like Ragdoll of Wild Wild Pussycats)
Creating an AU (Author Universe) or Crossovers must be thoroughly planned and thought over since any quirk is possible in the World.
Characters with Unique abilities like Ability Steal/Ability Cancel/Ability Boost might be interesting(even though these kinds of abilities exist in form of quirks with All For One, Aizawa, etc)
OCs with unexplainable quirks and multiple abilities are also favourable. Its best to give flexibility to the Character's powers to further build on their strength later(like Chakra Control, System, Wishes, Nen, etc). Try not to overdo common tropes like Sharingan or Infinite Mana as they can get pretty boring, very quickly.
QUIRKS EVOLVE INTO GREATER QUIRKS, CONSTANTLY MOLDING AND CHANGING, GO WILD!