Increased chakra reserves. This way, he could surpass Madara with a regular mangekyo sharingan due to the large arsenal of 5 elements and chakra and could defeat Orochimaru and many other strong shinobi. And also added more intelligence to him, so he would be the best Hokage on an equal footing with the first and second. Hiruzen would not spoil Naruto's life and could somehow solve the conflict with the idea of the Uchiha clan revolution in a different way.