The My Hero Academia power ceiling is a difficult one to gauge. At first, it seemed like the only way to achieve superhuman feats was to have a strength-enhancing Quirk. Now, however, there are all sorts of characters who have no such Quirk to speak of but can still achieve superhuman feats.
It would normally be a simple case to argue that MHA is a shonen manga and should therefore allow its character to train to be superhuman. However, the stark contrast between how gaining strength was depicted at the start of the series and later on can't be overlooked. It's okay to say that the series has shonen training, but there has to be an explanation for the contrast.
How Has the Power Scale Changed Throughout My Hero Academia?
At the start of the series, it seemed like Deku was only able to get about as strong as a normal human could get. The only times he's ever showcased superhuman strength have been when he was using One For All to increase his power. Beyond that, he was in peak physical condition at best.
His ability to keep up with his classmates also indicated that they weren't much stronger than the average human. Opponents like Katsuki Bakugō, Hitoshi Shinsō, and Shōto Todoroki seemed fairly vulnerable to his throws and punches provided he could reach them. Conversely, if one of them tried to punch him without using explosions or ice, they wouldn't do much more damage than the average human could. They might have had slightlyabove-averagee human physicality, but it wasn't always obvious what the upper limit was supposed to be.
The true power of non-strength Quirk users became more apparent during Deku and his friends' battle with Stain. The Hero Killer's Quirk was only supposed to allow him to paralyze those whose blood he'd ingested; no strength enhancement was implied. Despite this, he was strong enough to fight and take hits from Deku using five percent of One For All; this was on top of the attacks from Tenya Īda and Todoroki. From there, even as Deku got stronger, he continued to meet opponents who could withstand, keep pace with, and even overpower his ever-improving Quirk, regardless of their own Quirk.
Mirio Togata's strength, in particular, cannot be emphasized enough. His ability is to phase through objects, but he's also strong enough to punch Near High-End Nomus and send them flying. This is to say nothing of the time he beat nearly every student in Class 1-A by himself, including Deku, a boy with a Hardening Quirk, and a guy with a sugar-based strength Quirk. There's no way a normal human, however strong, should be able to do what he does.
How Strong Can Characters Get Without a Strength Quirk?
Any human with a Quirk in the MHA world may have always been capable of shonen training; the series is a Shōnen Jump manga, after all. They might have the same potential to train their bodies to give themselves superhuman levels of strength, speed, and durability as any character in any other battle manga. By that logic, somebody could theoretically train to become stronger than anyone presented in the series thus far, including All Might and All For One.
This is further backed by Shigaraki's unbelievable physical power. Even when his Quirks are erased by Eraserhead, he's still strong enough to fight multiple top-tier Heroes at once. His body may have been modified, but it still technically represents how strong a human can be without a Quirk to enhance their abilities.
As for why Deku never trained to get this strong, it might be because of his situation. At the start of the series, he was a Quirkless middle schooler who could only dream of becoming a great Hero. Once All Might trained him and passed on his Quirk, however, that dream became attainable and he could truly start working towards it.
It's also worth noting that One For All isn't actually a strength Quirk like many think. What it's supposed to do is take the accumulated power and Quirks of its previous users and pass it to the next person. However, even though none of the previous users had a strength Quirk, All Might was strong enough to create building-lifting tornadoes with his strongest punch. Needless to say, this isn't something eight people could normally do even if they were all in peak physical condition.
The main reason One For All is as strong as it is has to do with its fourth wielder, Hikage Shinomori. He spent most of his time with the Quirk training so that the next user would inherit all of his power; in the shot depicting this, he was shown to be strong enough to crack a boulder. He went through a shonen training arc so that his successors wouldn't have to.
Whatever the power ceiling for people is in MHA, it's probably not as low as the start of the series made it out to be. If a Hero trains hard enough, they can be just as strong as anybody with a strength Quirk. They probably can't get as strong as Shigaraki without some ideal Quirks or body modification, but they can still get pretty powerful.