Theory-Midoriya will save All-Might with One for All itself. (Spoilers)

To thwart Sir Night Eye's prophecy of doom that All-Might will die a gruesome and heart-wrenching death, Deku will, when his teacher's life is in peril, in a demonstration of admiration, reverence and sacrifice for his beloved idol, transfer One for All straight back to enable Toshinori to fight once again and save himself. All-Might will later in gratitude return the quirk back to his student and protege, Midoriya.