I have a NightMight villain au! It's kind of like the first one you suggested, but it gets sad
All Might gets injured, the two break up.
Nighteye, desperate to get All Might to stop, starts working overtime. Does everything he can to eliminate the villain who will ultimately kill All Might.
He gets captured during a villain raid. Here was his chance. He joins them, but he still needs to keep on the hero side. It would look bad if the Symbol of Peace's only sidekick turns into a villain.
So here he is, working as a hero in the day and a villain at night. His ideals are if there's no heroes or villains, All Might will have nothing to fight and he will have to retire.
He works tiredlessly to in hopes to achieve his dreams. All his hard work is in vain when All Might fights All for One and has to retire.
Nighteye only did all of this for All Might. He nearly stopped being both, but the "your next" line inspired him to keep being a hero, leaving his villain life behind.
Unfortunately for him, the villains weren't too happy about that. They give one more task. Take down a man named Overhaul.
It was a death mission. Their goal was to stop Nighteye for good.
The mission still happens. He dies knowing he was used by both sides.
I’M GOING TO SCREAM INTO THE S U N ACE THIS IS SO FREAKING SAD AND WONDERFUL AND BELLOW-WORTHY AND AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
this is it. the ultimate villain nightmight AU. now excuse me while i sob