Forced Retirement AU - Although Katsuki makes a near full recovery from his injuries, he's left with a heart issue that forces him to give up hero work. Izuku helps Katsuki learn to deal with his new found anxiety, frustration and low self esteem from feeling he'd failed - Izuku reminding him, much like Allmight, he gave up his dream instead to save others and that's what makes him a hero.
Katsuki is later invited to teach at UA by Toshinori, and eventually becomes the 1A teacher, finding passion where he didn't expect, and meeting kids who were inspired to become heros because of him.