Spoilers for Chapter 347 of My Hero Academia
Last week's chapter of My Hero Academia ended on a terrifying note for fans of the rabbit hero Miruko, but as the truth about Shigaraki's rapid growth was revealed in chapter 347, it may be a clue that fans of the latter are the ones who ought to be worried.
In chapter 346, Miruko engaged Shigaraki in one-on-one combat, with Miruko eager to show off what she's capable of with her new prosthetic limbs. The heroes have put together a solid plan to take out Japan's most wanted villain, a key component of which has been keeping Shigaraki's deadly quirks deactivated through the clever use of Monoma's copy quirk and Aizawa's Erasure. Shigaraki ended up scaring everyone, however, as he seemingly overpowered the Erasure quirk, generating a mass of gross fingers with which he retaliated against Miruko--a particularly dangerous threat, since Shigaraki's ability to use his Decay quirk is heavily tied to his "hands," no matter how many of them there may be. If Shigaraki were to be freed from Erasure's restrictions, the heroes have made it clear that all would be lost almost instantly.
As it turns out, however, Shigaraki's rapid growth wasn't a quirk at all; his body is just (allegedly) so advanced that he can grow new fingers and hands the way an ordinary person grows hair or fingernails. While the qualification as a quirk or not is a bit pedantic, this kind of body horror is pretty common amongst anime and manga villains, with the most famous case, of course, being that in the classic anime film (and its original manga) Akira. As a result of psychic powers running out of control, the antagonist Tetsuo is unable to maintain his form and becomes a pulsating mound of disgusting living flesh, radically increasing in size and posing a danger to all of Neo Tokyo. Much like Shigaraki, he abused this power as a weapon before it exceeded his ability to control it, and it's ultimately this overreaching and refusal to accept help regardless of how often it's offered that leads to Tetsuo's eventual demise.
Invoking this type of body horror in My Hero Academia immediately calls to mind Akira, a work which is so well known for it that even Western media like Rick and Morty will parody it. In anime and manga, characters who manipulate their own bodies in such unnatural ways almost always pay the price for it by losing control. With the reveal that this growth isn't some quirk ability, it automatically pushes what Shigaraki is doing outside of what's acceptable and "natural" for this world. By altering even his own flesh, Shigaraki is tempting fate at best and securing his own doom at worst.
Of course, Shigaraki's survival as himself is hardly anyone's goal here--All for One certainly doesn't care much whether that body is even remotely humanoid so long as it's powerful and it's his. The only one looking to get Shigaraki out of here alive is Midoriya, so how dark My Hero Academia gets may well depend on how long Shigaraki can hold it together.