My Hero Academia has set the stage for a big twist, where Izuku Midoriya could end up claiming the power of All For One. Read that again, carefully: we're not talking about the current arc of My Hero Academia's manga, in which Deku is clearly about to unlock the full potential of his One For All power. What we're saying is that the finer details and foreshadowing of My Hero Academia's story has set the stage for Izuku to ultimately claim the power of All For One from the villain(s) that have used it to terrorize the world for generations.
How's that going down? Here's the explanation (My Hero Academia manga SPOILERS Follow):
The latest chapters of the My Hero Academia manga have seen a comatose Izuku within the OFA realm, where the previous users have convened to reveal some key history about the power to Midoriya, who will be OFA's final user. We now know that One For All is a power meant for quirkless people and that it contains a portion of each previous user's consciousness and quirks inside of it. The predecessors test Izuku's heroic views by inquiring whether he will kill his rival Tomura Shigaraki, and eradicate the threat of All For One, once and for all. Of course, the War Arc with the villains ended with Deku taking on a new heroic mission to save Shigaraki from the grip of All For One, and that noble sentiment is what ultimately earns him the approval of OFA's users, and commits them to unlock the full potential of the power for Izuku.
However, what does saving Shigaraki from All For One mean?
Soon Deku will have the full One For All power (and all its quirks) at his disposal. As My Hero Academia has explained, OFA is the rare combination of a power-stacking and power-transfer quirk that's "alive" with the collective consciousness of its users. OFA has resisted All For One's attempts to absorb it - particularly in Deku's case. During the climax of the war, a direct battle of wills between Izuku and OFA's users, and All For One fueled by Shigaraki's hate, ended with Deku winning. His heroic nature and hope has been the core of My Hero Academia since the very beginning, and even OFA's users are now recognizing he's the most heroic disciple they've ever had.
All For One has threatened the world for generations. The War Arc showed (through the awakening of "Plus Ultra Shigaraki) that the power is dark and insatiable, and forever connected to the original villain himself. Izuku has already decided that OFA's purpose is not to destroy, but to save, so annihilating AFO is not his end goal. In chapter 305, Izuku's thinking starts to bend that way, as he considers every villain he's ever fought, and if there wasn't a better way to "save" them, rather than fighting.
The very beginning of My Hero Academia spelled it out the moment that Izuku sacrificed himself (with no quirk) to save Bakugo from a villain: heroic sacrifice is in Izuku's nature. If he's strong enough to repel All For One trying to absorb One For All, it makes sense that Deku using OFA at full power is strong enough to absorb AFO from Shigaraki.
It's been foreshadowed if OFA and AFO were to unite, it would be a doomsday scenario for society. However, that ominous prediction has always been from the perspective that All For One is holding both powers - if Izuku is the one who unites them within himself, it may be the very thing that explains this story of how he became the No. 1 hero.