The following content contains light spoilers from Season 6 of My Hero Academia, currently streaming on Crunchyroll. Although Dr. Ujiko has a relatively substantial role to play in the Paranormal Liberation War of Season 6, attentive viewers wonder where they have seen his face before; until everything suddenly clicks into place.
Fans are first introduced to this mad scientist under the guise of Dr. Tusbasa, who is the specialist that the juvenile and Quirkless Izuku Midoriya sought assistance from all those years ago, and it becomes quite clear just how meddlesome (and menacing) this two-faced doctor actually is. Many viewers have subsequently begun to question Dr. Ujiko's possible involvement in Deku's previous power-level problems (particularly considering how much he likes to reappropriate and repurpose Quirks for his villainous schemes.)
The Quirk Collecting Club
All for One and Dr. Ukijo have been besties for what must be close to a century by now (considering that Dr. Garaki disappeared about seventy years before the Paranormal Liberation War.) Both possessing a keen enthusiasm for stockpiling Quirks, these tyrants have essentially treated the world like their personal candy shop, where everyone's powers are just waiting to be taken home (and boy, do they have a sweet tooth).
While All For One conducts himself with brute force, Dr. Ujiko has taken the more strategic route by setting himself up as a respectable and established Quirk Specialist who can conveniently survey the power levels of individuals he meets along the way. Anyone who captures his interest could easily fall prey to the doctor's Quirk stealing procedure, such as with Oboro Shirakumo; why would little Izuku be any different? If Dr. Ujiko liked what he saw in the young boy, what would stop him from running some extra (extracting) "tests" or alerting his bestie about the lucrative situation?
Exploring Deku's Potential
If Izuku was not born Quirkless, then the identifying factors of his original ability is also up for debate. Bloodlines can play a significant role in Quirk inheritance, as each generation tends to share similarities with their forefathers' powers or, like in Shoto Todoroki's case, a perfect amalgamation of the two. Needless to say, not all Quirks are handed down directly from the previous generation, and it is therefore not guaranteed that an entire family will consist of similar Quirks (such as in the Jiro's case of musical masters.)
Deku's father, Hisashi Midoriya, possesses Dragon-like abilities which allow him to breathe fire, which presumedly created a great work opportunity for him that simply couldn't be missed (hence, his extended absence.) Deku's mother, Inko, has a limited centripetal gravitational field of sorts, which allows her to attract small objects toward herself (almost like the opposite effect of Ochaco's abilities.) If one assumes that Dr. Ujiko used his network to hand-select the best Quirks from unsuspecting victims, then it is also likely that All For One would get first choice when it comes to adopting new superpowered abilities. Two Quirks from the Symbol of Evil's arsenal stand out in particular when compared to Deku's parents' powers, which could have likely belonged to the young child before the underhanded thievery ensued.
Hardflame Fan: This potent move involves creating a blazing wall of fire around the user, that protects them while the enemy struggles in the overheated conditions. All For One acquired this skill from an anonymous source and is a believable upgrade of Hisashi's fire-based Quirk.
Air Cannon: Shigaraki favors this particular move during his battle against Endeavor at the beginning of Season 6, which is one of the well-used Quirks he inherited from All For One. This ability grants its user the chance to blow the opposition away (quite literally) by projecting a powerful burst of air with enough force to topple buildings. This Quirk could be the result of combining the centrifugal force of Hisashi's fire breathing with the gravitational energy of Inku's abilities (subsequently turning pull into push.)
Fan Theory Time
A particularly popular fan theory that has been making the rounds involves the other prominent prodigy of My Hero Academia; the one-and-only Tomura Shigaraki. Dr. Ujiko has clearly taken great delight in altering the young man's genetic structure, resulting in the omnipotent Awakened Symbol of Fear that is plaguing Season 6; however, many believe that the doctor has been dabbling with his DNA since way back in the Tenko Shimura days. The pair have a heart-to-heart towards the end of Season 5, which insinuates that Dr. Ujiko knows more about the young man's childhood than Shigaraki himself (which could have easily been a symptom of PTSD; or the doctor's preadolescent experimental interventions.)
This concept suggests that Deku's original Quirk was Decay, which could make sense considering fire is a destructive force, and Inko's abilities are rather hand-dependant. However, following this train of thought would require Shigaraki to have been the Quirkless one, or his original ability was swopped out during Dr. Ujiko's procedures; tinkering awfully close to getting sucked down the rabbit hole of conspiracy theories with this route! Alternatively, another favored fan theory debates that Izuku was the original wielder of the highly-lucrative Super Regeneration Quirk which, as fans are well aware, is one of the very reasons why the doctor's Nomu creations are so successful. Dr. Ujiko has not had access to these rejuvenating properties for a considerable amount of time and is still essentially in the testing stages of his zombie "babies;" meaning he could have easily stolen it from Deku during their visits.
Furthermore, if Deku did in fact possess a Super Regeneration ability, the Quirk's effects would likely be less obvious to be detected at a young age which is why he was considered to be Quirkless (although the doctor undoubtedly knew better.) Unless major wounds were sustained and suddenly healed, Inko would probably have no reason to suspect that her son had rapid healing powers, while the young boy was presumedly on the lookout for some kind of flashy superpower like Bakugo's. Izuku would have likely only presented his Quirk's true nature after sustaining a significant injury, which his overprotective mother prevented from occurring. Ironically, if Deku was capable of Super Regeneration, he would be able to wield One For All at 100% without much difficulty, and would not require Eri's involvement to get things done (at 100% capacity, that is.)