In My Hero Academia, protagonist Izuku Midoriya and Sir Nighteye had a rocky start, but their relationship was tragically cut short just as it was blossoming. Nighteye disliked Deku as All Might's chosen successor so much that their long friendship fell apart over the issue. Midoriya, on the other hand, proved himself to be more than deserving of the title over time.
While the two were bonding and Nighteye was seeing Deku's true abilities, the hero was fatally wounded while fighting the villain Overhaul. This was a tragic death in many ways; Midoriya had lost a mentor, and All Might had lost his closest friend, but Sir Nighteye's potential as a teacher for Deku was also lost.
Sir Nighteye Was Completely Knowledgeable About All Might In My Hero Academia
Sir Nighteye was All Might's biggest fan in MHA, with his knowledge of the #1 Pro Hero surpassing even Midoriya's. On top of that, he was the hero's sidekick for years and was his closest friend and confidant. With this knowledge, there's no doubt Nighteye knew exactly what All Might did to ascend to the top Pro Hero spot. Using this inside information, he could have passed his wisdom down to Deku and trained him in the same way All Might did.
Given that the two possess the same power, it would be great to train their strength in a comparable manner. Sir Nighteye would have been All Might's finest chance to mimic the fighting technique of the hero because he was never truly able to do so when aiding Midoriya. Although the apprentice is a skilled fighter, he has never received official combat training, therefore Nighteye could have developed his abilities using his understanding of the battlefield. He may have instructed Deku on how to use all of his strength, making him a more versatile and effective fighter overall.
The Depth of Sir Nighteye's Knowledge Might Have Aided Deku's Development
Sir Nighteye's experience explains why he would have made such an excellent teacher for Midoriya. Nighteye had previously proven himself to be an excellent mentor as well as being well-versed in the intricacies of combat. He previously mentored Mirio Togata and considered the young hero to be his greatest achievement. Given that Sir Nighteye's teachings helped Mirio become one of the University of Arizona's top students in less than a year, there's no doubt Midoriya would have been in good hands.
Due to being a well-experienced hero, Sir Nighteye could have also taught Deku about the side of hero work that goes unseen. He would have given Midoriya invaluable information about the values heroes should have, as well as the procedure of hero work. This could be everything from taking care of the paperwork, to how to approach a rescue situation or how to best present himself to other people -- all of which Midoriya doesn't have the best grasp on. Nighteye could have even taught Deku the subtler arts of hero work, such as spying on people, gathering intel and how to be as observant as possible.
Sir Nighteye would have ultimately been an excellent mentor for Deku in My Hero Academia, teaching him everything he's currently lacking and perfecting all the young hero's rough edges. With his guidance, there's no doubt Midoriya would have been a completely unstoppable force in current arcs.