Well, that sounds interesting.
Wild speculation:
At every odd level, the Fighter gets a Fighter feature. That's either something simple, like +1 to attack or +1 to damage, or something complicated, like an encounter or daily power in 4E.
Fighter Level 1:
Robustness: Get +1 hit point per level.
Enduring Strike: Once per day, when you hit with an attack, you gain temporary hit points equal to your level. While these temporary hit points last, you have advantage on Constitution Saves.
Fighter Level 3:
Fighter's Surge: 3 / Day, take an extra action.
Coordinated Assault: 3 / Day, when you hit an enemy, you can have an ally of your move up to 10 ft and make a single melee attack against the same enemy.
Fighter Level 5:
Extra Damage: +2 to damage rolls with weapon attacks.
Brutal Strike: Once per day, you can make an attack that deals +2d10 damage and knocks the target prone. If the attack misses, this ability is not expended.
Fighter Level 7:
Combat Precision: +1 to all weapon attack rolls.
Setup Strike: 3 / Day when you hit with an attack, the target grants advantage to all of your allies.
or something like that.
And a general Class Feature:
Fighter's Retraining: Each level, you can rechoose all Fighter Ability selections made.
Fighter's Retraining II: After each exntended rest, you may choose one Fighter Ability and select a different one for that level.
