So many fun ideas here. I can think of specific characters for whom each of the following would have been great payoffs.
Cohort
You’ve gained the services of a loyal cohort. Your cohort uses the Sidekick rules. When helping you don or doff armor, the time is halved. They can also use their own action to equip you with a shield.
Hero of the Games
When engaging in a prizefight or an athletic game like jousting or charioteering, you have advantage on your ability checks.
Heroic Endurance
Prerequisite: Proficiency in Endurance
You were pushed to the limits of mortal endurance and you persevered. Once per day after a short rest you may reduce your exhaustion level by your Constitution modifier. Additionally, you suffer no penalty from sleeping in light or medium armor.
Hidden Backstory
Choose a second Background. This might reflect a storied life, deep cover, dying and being resurrected, recovering from amnesia, or burying these skills due to some tragedy or personal vow
Remarkable Athlete
Prerequisite: Proficiency in Athletics
You gain a climbing and swimming speed equal to your walking speed, and your jumping distance is doubled.
Signature Item
You are known for a specific item, such as a sword, a signet, a helmet, or shield. This item becomes imbued with magic through your heroism over the course of your adventures. While you have 1-4 Fighter levels, the item is magical and gains a minor magical property (DMG 143) or sentience. While you have 5-10 Fighter levels, it also grants a +1 bonus to AC, +1 attack and damage, or another minor magical property. While you have 11-16 Fighter levels, this bonus increases to +2 or gain a +1 bonus to another of these categories. While you have 17-20 Fighter levels, this bonus increases to +3 or gain a +1 bonus to another of these categories.