sero

considering one of his quirks, his canonically stated drawback is dry skin. his tape is made of skin cells ! the tape being produced is literally just his dead skill cells (to make this make sense let's pretend his skin replenishes fast than average ,, like a LOTTT faster) i'm typing this and my dog won't stop barking im gonna crash out and so sigh this sounds gross .. but the aspect that makes his tape sticky could hypothetically be his sweat (it'd have to have biological differences but quirks do atone for that kind of stuff so) & if we considered that the stick was his sweat then hypothetically he could train to increase / decrease the stickiness by learning to control how much sweat was released in the creation (allowing for him to create a kind of rope if there was no stick at all) (sweat theory also takes into account the way his work dehydrates him)

hagaruke

so with the reveal of her literally being visible & her quirk just being light reflection hypothetically, if she learned to control that really well (to bend light), she could make light boxes where whoever she bent light around wouldn't be able to see (making their surroundings invisible) & further, if she could bend light, she could make lasers

todoroki

yeah the reason he can't control his fire as well as his ice is not soley bc he refused to use it but because it's a singular tempture quirk effecting the air around him (either freezing it or igniting it) & obviously fire just burns in oxygen & isn't containable which is why it always jsut does whatever the sigma it wants in the direction he's using it. & his ice is much more controlled because he can actually control what parts of the air he's freezing because it doesn't just burn

tokoyami + dark shadow

tokoyami quirk isn't sentiment they're conjoined twins & fumi has a quirk similar to whatever the fuck the name of that shadow jumping kid is but instead of going into shadows she rests in organs (but she can't rest in anyone else's because conjoined twin.. conjoined organs she'd die) & tokoyamis actual quirk (excluding the bird head..) is a real specific type of telepathy

aoyama

tldr: reverse photosynthesis he emits light right , & considering we don't know all the mechanics behind it & purposely ignoring its origin & the real reason for his tummy aches we can assume his stomach hurts because his body is eating at internal resources to produce said light (he's VERY dehydrated - which is a cause of tummy ache) because his body is using all the water in his body to produce the light (or like a shortage of electrolytes ) & his body isn't able to keep up with the water usage so he's constantly experiencing the side effects !!!!!!!1!1!1!1!1!

mido never quirkless theory

i think it would make sense if midoriya was never truly quirkless. following rlly basic theries of afo being his dad and the doctor stealing his quirk i think he had such a hard time and violent reactions bc he had a quirk factor but no quirk and his body was fighting the new quirk (s) bc it wasn't his og quirk but it was trying to take the spot if the og quirk,, and follow up here the reason he is the first one to devolp all the other users wuirks could be (if he had a dormant quirk, never taken just unknown like first) that his og quirk could've been some genre of a quirk enhancer mixed w afo & inkos quirk a quirk that pulled quirks to him (pulling all the mathers quirk to him & also potentially enhancing his ability for quirk analysis because his quirk was pulling on them & subconsciously feeding him information on how quirks worked

overhaul

this one might be like way canonically off, but also less of a quirk theory and more of a ?? he's goofy. his quirk literally just he can reassemble / disassemble everything like he can control atoms i think (matter blah blah) & he can obviously alter people's chemical makeup (or take parts out at least?) so literally.. he could've just removed people's quirk factors entirely whenever he dissembled them – he didn't need to experiment with eri to eradicate quirks (obviously, he couldn't have made the bullers without her dna BUT he didn't need her to remove quirks)

quirk # 1

QUIRK NAME : undecided (oxidation)
QUIRK TYPE : EMITTER (?)

QUIRK DEVELOPMENT :
BASIC QUIRK INFO : The user can accelerate the oxidation process in the compounds making up materials. Their emotions highly impact the user’s quirk — residual from their father’s empathic-based quirk. Quirk can only be used when applicable — if an object requires moisture to use, the user can only use their quirk on it if there is enough moisture in the air to accelerate the process. Quirk does not work on items made of compounds that do not oxidize (ie; plastics, ceramics, etc). The user has the best control in urban, moist/cool areas. Quirk is most effective when the weather is humid. At the time of the entrance exam, the quirk can be used, up to ~2.5 meters.


DRAWBACKS : User must have extreme control over their own emotions to ensure quirk controllability (ex; if the user is extremely overwhelmed they might accidentally activate their quirk, potentially causing combustion). Overuse of quirk causes the iron in the users’ blood to start oxidizing, causing the hemoglobin to break down & leading to anemic-like symptoms, hemochromatosis, joint/muscle inflammation, internal bleeding, and extreme overuse organ/brain failure. Breaks down users’ cells over time — leading to early death.
APPLICATIONS : (All at different levels of control/quirk growth, unorganized.) - Rust acceleration to weaken the structure(s) - Organic decay causes wilting, discolouration, and/or decomp. - Chemical decay-causing appearance / structural integrity to be damaged/weakened - Combustion (oxidizing oxygen..) causes heat/light (fire) to be released. Combustion comes with the risk of overwhelmed fires in areas where they quickly spread in the air & require immense concentration and control.
PARENT QUIRKS :

MOTHER : Minor chemical alteration quirk — can speed/slow rusting processes.
FATHER : Minor emotional manipulation quirk — can sense emotions up to ~7 meters, and “suggest” feelings. Suggestions only work off of users /own/ feelings at any given time.

quirk # 2

QUIRK NAME: something to do w bones
QUIRK TYPE: EMITTER
BASIC INFO: the user can accelerate & control bone growth and manipulate the shape of their skeletal structure, either reinforcing it for defense or forcing bone outward for offense. the bone only takes seconds to grow (timing depending on calcium levels) the bone easily breaks through skin (quirk side effect is naturally thinner skin making it easier for this aspect). bones may grow inwards to "thicken" defense.
DRAWBACKS : every use shreds muscle tissue as bones force their way through, leaving open wounds. (minor acceleration in healing. only heals when quirk is im use for 10mins) repeated overuse can cause severe osteoporosis, organ punctures, nerve damage, and eventual paralysis. extended overuse use may lead to a state where the skeleton becomes so brittle that a minor fall could cause multiple fractures (VERYYY low calcium levels)
PARENT QUIRKS : MOTHER : bone density manipulation can make bones temporarily harder + thicker FATHER : accelerated healing factor that only works on skeletal injuries.

quirk # 3

QUIRK NAME: boom blood or something QUIRK TYPE: EMITTER BASIC INFO: the user can ignite their own blood. (more specifics undetermined) the user can eject ignited blood via open wounds, or they can explode the blood directly out of their veins. (with extreme practice + control. not recommended.) ignited when the iron in the user’s blood undergoes a rapid chemical chain reaction, igniting into high-energy flame upon contact with excess oxygen. DRAWBACKS: death !!!!!! (the reaction permanently destroys all blood it consumes which leads rapid anemia + every use scorches the user’s veins and arteries from the inside, leaving irreversible scar tissue & eventually the circulatory system will become so damaged that the heart will no longer pump blood effectively, leading to organ failure) PARENT QUIRKS: MOTHER: minor blood ignition quirk, only minor sparks FATHER: pyrokensis