Theory-Midoriya will be the final user of One for All

Due to certain circumstances in the future, Deku will be forced to retain One for All and end up not passing it on to another user, taking the power to the grave:

• The whole world finds out that Deku's Quirk is originally All Might's and now a lot of people are actively trying to get the power from him, making it really difficult to find a suitable heir.
• One for All has become TOO powerful, to the point where if Midoriya tries to pass it on, his would-be successor would explode to Ludicrous Gibs because his/her body cannot handle the concentrated awesomeness.
• Midoriya ends up dying before he can find an heir.
• Midoriya will lose one for all before he can pass it on. (either by the quirk erasing bullets/the final battle between Shirigaki/All for One)