To pull it back into the discussion, I'm pondering going the magic user path for fighters in my next campaign. MUs used to be all arcanists until it was split into wizards, sorcerers, and warlocks based on origin of power.
I am pondering doing the same with Fighters.
Fighters are the technical weaponmasters who learn the martial techniques to mast the art of war. They might learn magic, runes, or psionics to supplement their martial training.
Champions are sorcerers of battle. Scions of great martial bloodlines, distant descendants of deities or monsters, or reincarnations of great heroes who have naturally occurring physical and mental prowess that translates to martial skill.
Hexblades are the warlocks of war. Great warriors bonded with patrons by pacts, these warriors are blatantly supernaturaly influenced.
Barbarians are warriors who channel spirits of nature, elements, ancestors, divinity, or tribal totems. These spirits grant a rage which strengthens and fortifies them.