Well, I agree, Fighter should not be a class, but a group of classes, like the Duelist, the Juggernaut, the Weapon Master, and so forth.

But to avoid skirting the question, he should be the colonel badass of the badass marines. With no magic, and only a couple of magic trinkets I think all adventurers should get (I don't believe in the Christmas trees, but I think it's not an unfair assumption that by level 10 everyone should have, say, one magic weapon or piece of armour, and a small Wondrous Item with a minor but useful effect), he kicks ass.

He gets dogpiled by a squad of gnolls with claws and fangs and knives, and comes out covered in blood... theirs. He holds up his shield against a dragon's fire and it deflects around both sides of him. He, by higher levels, can walk into a tavern and even without his chosen weapon and armour, is punching out the whole bar and wielding chair legs with ease. He's dueling Balors atop cliffs in the heart of the Abyss, effortlessly side-stepping magic assaults and parrying flaming swords with his own fairly mundane one. He's kicking in doors with such force that the door takes out a couple of armed guards even before he draws his sword to outfight the whole room.