I'm obviously far too late to the conversation, but in my mind you could make fighters more interesting if you made a few changes to the conceits of the game.
1. Combat Stamina. Has anyone here played Dark Souls? You have Hit Points that go down when you're hit, and aren't easy to replenish (just like D&D). And you have Stamina, which goes down whenever you take a strenuous action, but which replenish if you spend a few seconds not fighting.
Remember how early playtests of Next had martial dice? And for a while other classes got martial dice too? I kinda of liked the idea of everyone getting martial dice, but fighters get the most. It meant that in any prolonged battle, you wanted a fighter to hold the line.
2. Crack the Shell tactics. In normal D&D, enemies are perfectly healthy until they're dead. It'd be more interesting if most enemies had some sort of noteworthy defense. As long as it's up, they're hard to damage. Different classes (or builds) would be good at bypassing different types of defenses, or perhaps at take-downs if the defenses are gone.
These defenses might be simple things like heavy armor you have to break through, or a magic shield that reduces damage, or more complex like an intricate fighting style that you need the right countermeasure for. Make most offensive spells either built to remove defenses, or take down enemies once defenses are down, but not do both. Maybe you can't put an enemy to sleep unless he's already weakened and distracted, which encourages the fighter to wallop the guy a few times to distract him.