The following homebrew adds a modern option to the Fighter class in 5E TTRPGs. All of the core Fighter class abilities can stay rules as written; see the official 5E System Reference Document (SRD v5) for details. Note: This subclass has not yet gone through playtesting.
Bodyguard
Some fight for glory. You fight to keep others alive. Whether you’re a private security specialist, royal retainer, armored escort, or sworn protector, your duty is to anticipate threats and interpose yourself between harm and your charge. Your skill lies not in reckless offense, but in precision defense: Knowing when to move, when to strike, and when to shield another with your own life.
Level 3: Protective Instinct
You’ve trained to react instantly to danger near your allies.
Defensive Reaction. When a creature you can see within 5 feet of you is targeted by an attack, you can use your Reaction to impose disadvantage on that attack roll. You must be wielding a weapon or shield.
If the attack still hits, you can choose to take the damage in their place. You can use this feature a number of times equal to your Proficiency Bonus, regaining all expended uses when you finish a Long Rest.
Guard’s Stance. While you are within 5 feet of a creature you are protecting (an ally you designate at the start of your turn), that ally gains +1 AC as long as you are conscious and not incapacitated.
Level 3: Close Protection Training
You learn tactics for defense, movement, and quick reactions in dangerous environments.
* You gain proficiency in the Insight skill and one of the following: Perception or Investigation.
* When you roll initiative, you can move up to half your Speed to position yourself near an ally or cover, without provoking opportunity attacks.
* You can use Shields as improvised cover. As a Bonus Action, you can grant half-cover (+2 AC) to one adjacent ally until the start of your next turn.
Level 7: Intercept and Counter
You can intercept attacks and punish reckless enemies.
Intercept. When a creature you can see moves within 5 feet of an ally, you can use your Reaction to move up to 10 feet toward that creature. If you end this movement within 5 feet, the creature’s movement stops immediately, and its Speed becomes 0 until the start of its next turn.
Counterstrike. If you use your Defensive Reaction and the attack against your ally misses, you can immediately make one weapon attack against the attacker as part of the same Reaction.
Level 10: Layered Defense
You’ve mastered using your environment and awareness to cover others.
Cover Awareness. While you and an ally are both behind the same cover, that cover’s bonus to AC is increased by +1.
Team Evasion. When an ally within 10 feet must make a Dexterity saving throw to avoid an effect that also targets you (such as an explosion or area spell), they gain advantage on the save if you succeed on yours.
Mobile Guard. Standing up from prone costs you 0 movement, and allies within 5 feet can do the same by using 5 feet of their movement.
Level 15: Unyielding Protector
Your endurance and focus allow you to absorb devastating attacks.
When you or an ally within 5 feet take damage, you can use your Reaction to reduce that damage by 1d10 + your Constitution modifier + your Fighter level / 2 (rounded down).
When you use Second Wind, you may divide the healing between yourself and one ally within 5 feet as you choose.
Level 18: Last Stand
You refuse to let your charge fall. As a Reaction when an ally within 10 feet is reduced to 0 hit points, you can move up to your Speed toward them (without provoking opportunity attacks) and choose one of the following:
* Shield Their Fall. The ally instead drops to 1 hit point, and you take damage equal to the attack’s damage roll.
* Tactical Revival. If the ally falls unconscious, they immediately regain hit points equal to your Fighter level and stand up as part of your reaction.
Once you use this feature, you can’t use it again until you finish a Long Rest, unless you spend a Hit Die at the end of a Short Rest to recharge it.