There's a few ways to make fighters a little more compelling:
- Let them be the only class that can use heavy armors from the get-go, all the others (and yes, Paladins also, they have enough already) gain maximum medium armors.
- Indomitable is a joke of a feature. At the time a Rogue can decide to roll a 20 or just decide to hit once per rest, at the time a monk has proficiency on all saves and can straight up remove some effects on themselves and reroll saves 20 times per rests, the fighter get to do it 3 time a DAY ?
No...just no. Make Indomitable be ''replace a save with a 20'' 3 times a day. Done. And yes, it applies to Death Save also.
- A bunch of other class have access to fighting styles, at least make fighters have more!
- Rogues get to use Sneak attack on AoO, Paladin can Smite on AoO, Barbarian gain their bonus damage on AoO, Caster can (at the cost of a Feat) use Spells on AoO....let the goshdarn fighter use its whole attacks sequence as a AoO, making it actually dangerous to turn your back on a veteran warrior!
- Cantrips progression are at 5th level, 11th level and 17th level. Make extra attack scales the same way and give the fighter a proper capstone.
- Speaking of capstone, just give them 1 legendary action per round to make a single attack, move half speed or shove. Basic and efficient.
- Give options for tables playing without feats for those extra ASI! Be it just: ''pick a save proficiency'' or ''gain Expertise in a skill''.
- Give actual features to the Champion. ''Half proficiency to untrained skill check that uses 2 stats: one with only one skill and one with none'' ? C'mon!
Advantage on Initiative and Athletics and extra jump distance should be the minimum. Improved crit? Ok, but boring.
Add ''Once per round, if you roll the maximum on a damage die, roll an extra die on the same size and add it to the total''.
So much possibilities without reinventing the wheel.