Supposedly, Eri has a quirk, Rewind, that allows her to rewind an individual's body to a previous state. She used this, for example, to undo the damage Izuku did to his own body when he used One for All at 100%. Could Eri restore the One for All quirk to All Might and also cure him from his injuries done by All for One? Or are there any obstacles for something like this to happen?
That's a highly speculated topic in the fan base. Since we don't know much about both quirks, we can only judge from what we know.
Since Eri was able to restore wounds on Izuku, we can say that she can restore All Might's wound too. Eri can even rewind someone to their non-existence which, of course, means death.
We know that quirk bullets can make a person quirkless. It's also important to note that these bullets only change your quirk status, not the whole body.
Thus, you can assume that with experiments or by risking All Might, Eri can reverse him to a previous state where he is at his "might".
Now, these are my own thoughts. I think Eri can reverse All Might's quirk because of how One for All works. If a One for All user passes the quirk to someone else, what's left are just the embers. So, I think she can only restore those embers but not his power.
Eri's quirk is the most broken quirk we ever know. I think the mangaka knows this too so he won't use her because of this. It can destroy every buildup the manga has ever done.
From what I understand so far, Eri's quirk is capable of reversing an individuals body back to any point in time (from the individuals current state to any point before they existed). This means that she could ideally heal any person from any possible mortal damage because her ability can simply reverse the persons body back to a time before such an injury was sustained (just like she did with Midoriya Izuku). So its possible for her to heal the injuries that All might got from various villains.
The complicated part is If she can restore All might back to a time where he possessed One for all. Before I start with how that question can be answered, we need to understand the concept of paradoxes. Time moves linearly and is usually unchangeable, but in Eri's case, her ability affects time very subtly, Eri can take someones body back to any point before they were born, and in a way, that is a "time-related ability".
Usually, time related abilities cause a ripple effect which I personally named as "Time relapse" (the consequent result of an action due to its own distinct time difference). However, Eri's "Rewind" does not trigger a "Time relapse" and that either shows that:
(1) The complete nature of her quirk is still unknown Or (2) The mangaka has not completely thought up those aspects as of now. or (3) I'm just way overthinking things lol!
Paradoxes are logical possibilities which create apparent contradictions towards already known facts. I bring this up because All might has already passed the one for all ability to Midoriya, and so Midoriya is the current wielder of one for all. If Eri reversed All might's body back to a time where he has One for all then that would mean that there are 2 torch bearers of one for all. However, we already know that there can only be one wielder of One for all at any given time. This therby causes a paradox in that timeline and contradicts what we've already been told. Therefore it will be Impossible for her to take All might back to a time where he possessed One for All.
I don't know how paradoxes will be enforced in the "BNH Universe" but I'm guessing that no matter what she does, there will be "some force" preventing her from restoring One for All back to All might.
Eri's Rewind Quirk attracts so many possibilities and paradoxes that conflict with the overall logic of the anime as a whole. However, that is the route the Mangaka chose because her name itself ("壊" + "理") means to "break reason".
After starting the story from the beginning again, I've come to the conclusion that, while what you are saying might be possible... the most likely outcome is that All for one steals her quirk and restores All for One to his prime. I know he meant to pass things on to Tomura, but I just find it highly unlikely that Deku will never fight a full powered All for One.
I think the main problem is the fact you need to look at this time thing in many ways. A quirk works in many diffrent ways, and so does time travel. Eri’s quirk can control the time of someone’s body, which allows her to use her quirk without effecting the time stream. Of course, if she can get rid of a quick, she’s using her quirk on another scale. She can use her quirk to not only reverse one persons body/ other stuff, but as well as the humans evolutionary state. For example, she could go to a person, and in theory, turn them into apes, from which we have evolved from.As for all mights quirk, he won’t be able to give that back because that was something much diffrent. Of course deku could always give it back, but I think Eri’s ability follow diffrent strands of time, but she would most likely be able to heal the damage all might got.