Among all classes the Fighter is the one that has – since the dawn of the game – lost most of its “mojo”. At least in previous editions, you could argue that other “fighting-man” classes required stepper prerequisite and higher amounts of XP, but D&D 3rd brought an end to that.
The Fighter’s “thing” in 3.5/Pathfinder became Feats. The problem with that is:
1) All classes can buy feats;
2) Feats usually are fixed benefits;
3) Fighter-only feats are rare and usually limited to just more fixed bonuses (i.e. boring)
The D&D 3.5 Player’s Handbook had – for Fighters – only the Weapon Specialization feat tree (the net result is “amazing” +4 bonus to damage… with just ONE weapon).
The D&D 3.5 Player’s Handbook II and Complete Warrior helped a little the poor Fighter, although most of the new martial feats also could be bought by other classes – the exception were usually those that required Weapon Specialization. However, to make things further worse, some core classes like the Psychic Warrior and the Warblade (besides a few prestige classes) could buy Fighter-exclusive feats. As a consequence, the Fighter kept its backseat position among martial classes.
The problem is old: the Fighter’s niche is basically hit things and resisting damage – all future martial classes did the same thing and something more. It’s hard to balance this without also giving the Fighter something “extra” (and personally I also find hard doing that without removing the Fighter’s classic status as an entry-level class – the type of character that you encourage new players to try).
If you’re not bound to a traditional D&D/Pathfinder structure, there’re a lot of good d20 games out there that give excellent ideas to improve and personalize the Fighter’s niche – the Iron Heroes feat system and its Men-at-arms class (I remember trying to use the layered feats of this game exclusively on my Fighters, but it required some work). In my home campaigns I created a new version of the Fighter with its own signature mechanic, but that cost me the class’s delicious original simplicity (already bruised by the feat mechanic).
So, in the end, if you want to protect the Fighter and keep things as simple as possible, creating more exclusive feats are the way to go in my humble opinion. Pathfinder helps a little in this regard by breaking the old misconception that class-exclusive feats are “wrong” (at least a few years ago creating that kind of ability would grant you unanimous rebuttal from most brazilian forums that I frequented). There now in the Core Rulebook itself a number of cool exclusive feats (even though most are high-level ones) like Penetrating Strike and Critical Mastery.
The secret with new exclusive feats (again IMHO) is that they shouldn’t steal the light of other martial classes. Because of this I'm usually against new feats that are basically combat maneuvers or ways of using a particular weapon to execute a cool attack. Some classes – like the barbarian for example – should also do that kind of stuff. That’s why I prefer feats with a more abstract nature these days, things that tinker with the combat rules’ inner mechanics.
With those thoughts in my mind here are two new feats. The first one was actually inspired by an Old School discussion (I just can’t remember where), while the second is a meekly attempt to reconcile armors both as abstract defense (AC) and damage reduction (kind of).
You learned to use your strength to deflect blows with your shield.
Prerequisites: Str 13+, shield proficiency, fighter level 3rd.
Benefit: While using a light or heavy shield, you can substitute your Strength modifier for Dexterity to determinate your Armor Class.
Your resiliency and skill with heavier armors allow you to diminish the impact of otherwise lethal blows.
Prerequisites: Medium or Heavy Armor Proficiency, endurance, fighter level 8th.
Benefit: While wearing medium or heavy armor, you gain a margin of protection equal to half the armor bonus granted by your suit. If you're hit, but the attack roll is within this margin you suffer nonlethal instead of lethal damage. If the original attack causes nonlethal damage, reduce it by half.
This feat doesn’t work if you’re flat-footed.
Example: Heltz, a human fighter, has a full plate +1 (armor bonus +9, ignore the enhancement bonus), a large steel shield (+2 shield bonus) and has Dexterity 11. His AC is 21. His margin of protection is 5 (the full plate's armor bonus divide by 2, rounded up). So, if any attack against Heltz hits AC 22-26, the lethal damage will be converted to nonlethal damage.