WARNING: The following contains spoilers for My Hero Academia Season 5, Episode 3, "Clash! Class A vs. Class B!" now streaming on Crunchyroll, Funimation and Hulu.
In Episode 3 of My Hero Academia's fifth season, Midoriya talks to All-Might about a dream he had that allowed him to see the first user of One For All's past. While All-Might also was able to have these visions, Midoriya's are slightly different, as the first user was able to directly address him. All-Might and Midoriya are both confused by this, but since the first user mentioned something about a "singularity," Midoriya wonders if this is connected to the Quirk singularity doomsday theory.
This theory claims that as Quirks are mixed between parents and passed down through generations that they will eventually create more powerful Quirks that no one should be able to control. While this is not the norm for My Hero Academia yet, there are several characters who seem to prove this theory is plausible, despite some of them having mastered their Quirks.
Eri - Rewind
Eri is the most obvious one on this list, as she has yet to master her Quirk and still needs to be under observation because of how dangerous it is. Rewind is ridiculously powerful, as it has the ability to rewind someone's biological clock. There is potential with this Quirk, as Eri can undo someone's serious injuries by reverting their body back to how it was before they were hurt, as she does with Midoriya.
However, she lacks control of this power, so she can also rewind a person out of existence by accident. She is a child, so she could learn to better control this power later on; however, her Quirk only applies to living beings, so it'll be difficult to control such a powerful ability without risking the lives of others, proving the theory correct in the respect that some Quirks are getting harder to manage.
Tomura Shigaraki - Decay
Shigaraki has been one of the most prominent villains throughout the anime, and his Quirk is devastating. Decay allows Shigaraki to disintegrate anything he touches with all five fingers. He was able to seriously damage Eraser Head's elbow with this ability, and he removed Overhaul's arms with it as well, rendering him unable to use his Quirk in the future.
While Shigaraki has control of his Quirk, Decay -- like Rewind -- must've been difficult to master without risking the lives of others, again proving the theory right about how some Quirks are becoming more difficult and dangerous to control with each generation.
Shoto Todoroki - Half-Cold Half-Hot
When Midoriya is discussing this theory with All-Might, clips playback of the Todoroki family, and there is a focus on Shoto when Midoriya remarks that, "they will create more powerful, complex Quirks someday that no one will be able to control." While Todoroki knows how to handle his Quirk, he is proof that when two parents with Quirks have a kid, there is a chance that the child will inherit a hybrid of both their abilities, and it will surpass their own.
Along with that, Todoroki's Quirk comes with downsides, as his body will suffer frostbite if he overuses his ice powers, and he'll suffer from heatstroke if he overuses his fire powers. He's found a way to navigate this by alternating between his abilities, but he's another example of how Quirks get more powerful with each generation, even as said Quirks develop setbacks that make them difficult to control.
Kai Chisaki - Overhaul
Another major antagonist in the anime, Chisaki's Quirk is one of the most horrific to date. Like Shigaraki, he just needs to touch something or someone to disassemble it entirely, easily killing Magne. He can also reassemble something or someone with his touch, reviving Kendo Rappa seconds after he destroyed his upper body and even merging his own body with Shin and Rikiya.
A Quirk like this practically gives Chisaki control of all matter, making the Quirk difficult to fight against. As someone who thinks Quirks are a plague on society, Chisaki likely believes in the Quirk singularity doomsday theory as well, even though he is proof that some Quirks are becoming too powerful for the world.
Twice - Double
While Double may not seem as powerful as some of the aforementioned Quirks, it is a highly valuable Quirk for the League of Villains and poses a huge threat to heroes. Double allows Twice to make a copy of himself, someone else, or an object, and if he makes a duplicate of a person, that copy retains the original person's Quirk, personality and autonomy, so Twice cannot control what they do.
Not only does this lack of control support the theory, but this Quirk has also taken a huge toll on him mentally, as he created a clone army of himself in the past that turned against him. While he fought them off, to this day, Twice is unsure if he's the original Twice, proving that some of the most powerful Quirks can torment their users in unexpected ways.
Fumikage Tokoyami - Dark Shadow
Tokoyami is a student with immense potential, placing third during the Sports Festival and interning under Hawks. It's no surprise that his Quirk, Dark Shadow, is also incredible; however, it is far from easy to control. While most Quirks act as abilities, Dark Shadow is a sentient being that Tokoyami can control at times; however, when it is dark out, Dark Shadow grows more powerful and out of hand, as seen during the Forest Training Camp Arc.
This can cause Tokoyami immense distress and lead to Dark Shadow harming others. While the aspiring hero has worked on better controlling Dark Shadow, a Quirk that is sentient and can act on its own supports the Quirk singularity doomsday theory.
Denki Kaminari - Electrification
Kaminari may not be the best of his class in terms of grades, but his Quirk is impressive and powerful. Electrification means Kaminari can generate electricity and fire bolts of it at great distances; however, without a support item, Kaminari cannot control where his electricity is discharged. Plus, he can exceed his wattage limit, which leaves him in a stunned and vulnerable state where he cannot use his Quirk for some time.
While he has his support item and has trained to better handle large amounts of electricity, this Quirk is near impossible to control on its own, thus proving the Quirk singularity doomsday theory correct.
Izuku Midoriya - One For All
Unlike most people with Quirks, Midoriya was not born with his; however, he did inherit it from All-Might. It is one of the most powerful Quirks in the anime, so it is inherently going to be difficult to control, with Midoriya only managing to handle 20 percent of its power at the time of this writing. Along with that, One For All can seriously harm the user, with Midoriya suffering from several major injuries before even becoming a Pro Hero.
Furthermore, this Quirk is evolving over time, as it developed a way for Midoriya to be in direct contact with the first user, something All-Might couldn't do. The difficulty of this Quirk, as well as its ability to evolve over time, supports the Quirk singularity doomsday theory, which claims that Quirks will evolve over time to become more powerful and uncontrollable.