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.
Space Marine
You are a soldier forged for combat beyond the bounds of any one world. Clad in advanced armor and trained in void operations, you embody the ideals of precision, endurance, and adaptability. Space Marines are the vanguard of humanity’s expansion into the stars… or the last defense against what lurks among them.
Level 3: Powered Combat Suit
Your training includes the operation of an advanced exosuit or power armor that enhances your strength, mobility, and survivability. You can don or remove this armor as an Action.
While wearing it, you gain the following benefits:
Enhanced Protection. Your AC equals 16 + your Dexterity modifier (maximum +2), or your current armor’s AC, whichever is higher.
Augmented Strength. Your carrying capacity is doubled, and you count as one size larger when determining the weight you can push, drag, or lift.
Life Support. The suit provides up to 1 hour of oxygen, limited radiation shielding, and resistance to extreme heat and cold (per the environmental hazards rules).
You can repair or recharge your suit during a Short or Long Rest using a Tech Repair Kit.
Level 3: Tactical Targeting
You gain access to a smart-link targeting system integrated with your suit or visor.
When you make a ranged attack with a firearm, energy weapon, or thrown object, you can add your Intelligence modifier to the attack roll in addition to your Dexterity.
If you already use Intelligence for that attack, you instead gain advantage on the first attack you make each turn against a creature you have targeted with this system (marking them as your Target Lock until the start of your next turn).
Level 7: Adaptive Training
You can reconfigure your suit and combat tactics on the fly. As a Bonus Action, choose one of the following combat protocols, which lasts for 1 minute or until you switch again (no action required at the start of your turn). You can activate this feature a number of times equal to your proficiency bonus per long rest.
Assault Protocol. Your movement speed increases by 10 feet, and you can Dash as a Bonus Action.
Defender Protocol. You gain temporary hit points equal to your Fighter level and resistance to bludgeoning, piercing, and slashing damage from non-magical attacks.
Support Protocol. You can use the Help action as a Bonus Action, and allied creatures within 10 feet of you gain +1 AC while you remain conscious.
Level 10: Zero-G Maneuvering
You have mastered combat in microgravity and vacuum.
You gain a Fly Speed equal to your walking Speed while in zero-gravity or while using jump jets integrated into your armor.
You can’t be knocked prone or pushed unless you wish it while in low or zero gravity.
When you make a melee attack after moving at least 10 feet toward a target, you deal an extra 1d8 Force damage as you deliver a kinetic-boosted strike.
Level 15: Overcharge System
Your exosuit or weaponry can enter a dangerous overdrive state.
As a Bonus Action, you can overcharge your systems for 1 minute. While active:
* You deal +1d6 damage on all weapon attacks.
* Your movement speed increases by 10 feet.
* When you take the Attack action, you can make one additional weapon attack as part of that action.
When Overcharge ends, make a DC 15 Constitution saving throw. On a failure, your systems short-out, giving you one level of Exhaustion until you complete a Long Rest.
You can use this feature once per Long Rest, or again by expending two Indomitable uses (if available).
Level 18: Orbital Drop Assault
You can descend from orbit or leap from extreme altitude, turning your arrival into devastation. Once per Long Rest, as an Action, choose a point you can see within 60 feet. You rocket into the air, momentarily disappear, then crash down like a meteor. Each creature within 20 feet of the impact must make a Dexterity saving throw (DC 8 + your proficiency bonus + your Strength modifier).
On a failed save, a creature takes 8d10 Force damage and is knocked Prone.
On a successful save, it takes half damage and isn’t knocked prone.
You remain standing in the crater, immune to the damage from your own impact.