One of the things that I've noticed about d20 combat is that it becomes desperately dull. Round to round, there's really nothing for d20 melee classes to do. Deny it if you will, folks, but there just aren't enough interesting things to do. The games that try to fix this problem give all sorts of powers and special abilities to fighters, thinking that making them more like spellcasters will make them more interesting. This, of course, just leads to homogenization and people complaining that they didn't want to play a spellcaster. The problem, of course, isn't with the classes, but with combat itself.
One solution? Houserule in melee weapon reach. Almost all combats in real life are decided by whether you can close in to hit your opponent with that critical strike, but if you can't close that gap then you're screwed. That's why the spear was the common weapon back in medieval times, not the sword. It was cheap, deadly, and the reach to ensure that people couldn't just close without some issues first. The strategy then shifts from trying to hit someone as hard as you can to getting in close and staying there.
These rules assume 3rd edition/Pathfinder/Star Wars SAGA as the base rules being modified. These rules haven't been playtested yet, this is merely a thought exercise.
Whenever you become adjacent to an opposing character the two of you must make an opposed Dexterity or Acrobatics+Base Attack Bonus roll. The character with the longest weapon gets a +3 bonus to this roll. The winner of this roll gets a bonus to attack and damage rolls, equal to triple the difference of weapon length.
At the beginning of each round all characters engaged in melee must make this check once again, with the winner of the previous round getting a +1 bonus per round that he has had advantage.
The following actions are modified as to fit with this system.
Charge: You must make the engagement check as part of the charge, but you gain the weapon bonus as if you'd already won the engagement. If you fail this roll your opponent's next attack against you is an automatic critical hit.
Grapple: The grapple move ignores all advantages and disadvantages. If the target of the grapple is wielding a longer or longest weapon the attacker gets a +3 bonus to his roll.
Sunder: You don't get the bonus to damage rolls from advantage when sundering a weapon, although you do keep the bonus to attack rolls.
Disarm: Keep the advantage bonus. If a character is trained in Bluff he uses that bonus instead of his Base Attack Bonus, whichever's higher.
1. Shortest: Hand, dagger
2. Short: Short sword
3. Long: Longsword, Axe,
4. Longer: Glaive, Spear
5. Longest: Bow, crossbow
Example: Xenith, a 4th level paladin with a longsword, engages Xellus, a 3rd level rogue with a dagger. Xenith has a bonus of +7 (4 BAB+3 Longer weapon+1 Dex mod) vs. Xellus's +7 (2 BAB+5 Acrobatics). Let's say, for the sake of the argument, that Xellus wins the roll. He gets a +6 to his attack and damage rolls because his dagger is 2 weapon lengths in difference from Xenith's longsword.
However, the next round they have to make the check again, although Xellus has a +8 bonus because he had won the previous engagement's roll.
Whenever an attack roll is made two things must happen: the target must declare what part of his body he leaves open. The attacker then declares what part of the body he's aiming for. If hit the target must make a Will Save with the damage dealt as the DC. If the save is failed, an additional penalty happens to the target:
Legs: Target falls prone.
Arms: Target drops whatever he's holding on that side. If a shield is strapped to his arm he receives a -3 penalty to using that arm for the next round.
Torso: Target takes a cumulative -1 penalty to all physical rolls until he takes a full-round action to recover.
Head: Target loses his next move action.
Give these rules a try, and let me know what you think of them. I'll be trying them tonight, and I'll post about what I find.