“When your power eclipses mine I will become expendable. This is the Rule of Two: one Master and one apprentice. When you are ready to claim the mantle of Dark Lord as your own, you must do so by eliminating me.“
Darth Bane to Darth Zannah, Star Wars
So far in the story of My Hero Academia, the vestige of All For One has taken over the body of Tomura Shigaraki. It turns out that All For One’s idea of a successor is not someone who would replace him but someone who would become a new body for his essence to inhabit. According to Hawks, despite being the most sinister supervillain in Japan, All For One is not truly capable of feeling true hate. This could be why he was unable to steal the One For All quirk because he lacked a suitable amount of hate to fuel his willpower. Therefore, All For One took Shigaraki under his wing so he could nurture and encourage his hatred. When Shigaraki’s hatred was strong enough, he would be given the All For One quirk. With Shigaraki’s hatred combined with the vestige of All For One, All For One himself would have a strong enough willpower to override the vestiges of One For All, which will allow him to steal it. Based on the vestiges of One For All, that has been All For One’s plan from the very beginning.
However, I do not think everything will go as All For One has planned. All For One’s foresight is frightening and impressive, but it clearly has limits such as when he admitted that the Paranormal Liberation War did not go exactly as he planned. In the vestige world, All For One himself hinted that there is a possibility that Shigaraki may be able to usurp him as the true wielder of the All For One quirk. I think there is a real possibility of this happening. As mentioned before, All For One is evil, but he has no true hatred in him while Shigaraki is overflowing with it. That means that Shigaraki has a way to fuel his own willpower that All For One lacks.
I think Shigaraki’s hatred is going to get a serious boost that will not only allow him to usurp the All For One vestige, but also complete his transformation as a supervillain. It has long been theorized that Shigaraki received his Decay quirk from All For One when he was a child. This was hinted in a flashback when a mysterious man that resembled All For One returned Shigaraki to his family after finding him in the streets. That would mean that Shigaraki was originally quirkless like Deku was before receiving One For All, which would be yet another parallel these two characters would share. This would also mean that All For One was indirectly responsible for the tragedy that Shigaraki suffered when he killed his family with the Decay quirk. In other words, All For One orchestrated Shigaraki’s trauma in order gain custody of him. I would not be surprised if All For One used some kind of telepathic quirk to influence the minds of Shigaraki’s family in order to further increase the trauma Shigaraki would endure. If this is true and Shigaraki finds out, that would definitely give him an even bigger motivator to turn on All For One. It would also make Shigaraki an even darker supervillain than before because his family abused him, hero society abandoned him, and his own master used him and destroyed his life. All of this would make Shigaraki even more hate-filled and may make the sliver of good Deku glimpsed in Shigaraki evaporate.
While the One For All quirk has a symbiotic relationship between its wielder and the vestiges of its previous users, the All For One quirk acts more like a parasite where the wielder and the vestige fight over control of one body. Here is the reality of Shigaraki’s situation. Either he overthrows All For One and gains control of his power or All For One overthrows him and gains control of his body. There is no middle ground.