Here's Fighter as a Barbarian subclass.

3rd: Well of Strength
1/SR Second Wind, but also activates Rage for free if you didn't have it active.

6th: Primal Surge
1/SR Action Surge, but also activates Rage for free if you didn't have it active.

10th: Indomitable Rage
1/Rage: Reroll failed saving throw, add any Brutal Critical dice to roll (so eventually 1d20+mods+3d6 from 17th level)

14th: Extra Attack (3)
Extra Attack...again.