Hey, a crazy yet plausible version of my earlier theory just came to me. What if during the final battle, AFO finds himself striking Deku without intending to and wondering what caused it. It turned it was because of Tenko's subconscious still inside him. Due to Deku having thrown all of OFA's Quirks into him (along with him having claimed Danger Sense), Tenko was in turn able to transfer his own vestige to Deku, along with not just OFA's Quirks, but all the Quirks inside AFO that could be used for good (including Super Regeneration that completely heals Deku's arms). This would allow Deku to save Tenko, regain his Quirk and enable him to finish off AFO for good all at once.