Whether or not you're familiar with my combat system - and there are many posts about it, plus these the newbie should start with - you may be able to fit this idea in with your present system.
One of my house rules is that when a '1' is rolled on the 20-sided attack die, the weapon the player has is dropped. Apart from determining if the weapon breaks or not, the weapon ends up in one of these hexes, determined by a 8-sided die:
Admittedly, I've considered a version using a 30-sided die, but it is a bit overwhelming:
It changes the odds of the weapon falling at the user's feet from 1 in 4 to 1 in 5 ... and definitely increases the annoyance of dropping a weapon by having it fall two hexes away 40% of the time. It is, unfortunately, a difficult table to memorize, and thus the reason for using the simpler version.
Having combatists drop weapons, however, I find myself thinking of a different matter entirely. And I'm surprised I have had a player ask me about it. Strangely, it's taken for granted that you'll have to go get your weapon, wherever its fallen. I have had players pick up someone else's weapon in a moment of combat - particularly an axe or dagger that makes a missile weapon - but I've never had anyone suggest kicking away a weapon that's hit the ground.
Take the following situation:
White has dropped his own sword in the hex he's in, and it's Blue's move. Normally, blue would attack White, White would recollect his weapon and attack Blue, and so on.
Let's suppose, however, that Blue decides to either kick away White's weapon, or possibly hook it with his own sword and sweep it behind him. We can assign numbers on a 12-sided die to determine his success, so that if Blue manages, he moves the dropped weapon to hexes 1 through 12.
What determines success?
We can easily assign the weapon a given AC ... for some, this might be high. For me, I'd say the AC is no better than 10. There must be something that stops people from hitting a creature without armor or shield (AC 10) ... that something must be the weapon the defending creature is using.
We could modify the roll easily. The defender's dexterity ought to come into play, to stamp on the dropped weapon before it can be moved away. The attacker's dexterity ought to be ignored, as it is ignored for all combat attacks. The attacker should have to choose to "attack" the dropped weapon instead of the defender, so the defender wouldn't take damage no matter the success of the attacker. If the dropped weapon were in the same hex as the defender, possibly -2 could be included because of the reach ... but perhaps not. If the dropped weapon were not in the same hex as the defender - but in reach of the attacker - Blue should be able to easily move to the left or right and have a just about automatic chance of kicking away the weapon. If it were in the hexes behind White, that might depend on how much move Blue has. So that would mean there was no comparative situation vs. which the -2 modifier ought to, or ought not to, be included. Might as well just call the dropped weapon AC 8.
It could be a good thing to go for the weapon and not the defender ... after all, if you get the weapon far enough away, that takes care of that defender for a round. And if the combat is on the edge of a cliff ... goodbye magic sword.
The other idea I had for combat ... and this has nothing to do with the above ... involves a rather cliched scene where someone about to be hit is - in the nick of time - snapped out of the way by a friend. The only one I can think of right off is the scene in League of Extraordinary Gentlemen, where Quartermain pushes Sawyer out of the way in the nick of time, keeping him from being hit by a roof tile (or whatever Hyde throws). I liked the movie - I know people don't, due to the sad, inconsistent special effects and other reasons - but try to concentrate on the actual event.
What else might a back-up do? Taunt the enemy? Jab a weapon at the right moments, not near enough to hit but enough to harrass the enemy into missing a defender? While the apparent positions for the game may seem static, we must assume that people are floating all over the hexes they're defending ... perhaps that gives opportunities for a back up to make themselves especially useful.
What would it offer? Would it simply mean a 1 better armor class could be added by someone behind the fighting creature? A two better armor class?
It would have interesting possibilities. If +2 AC, then a line of defenders two deep would certainly be harder to take down than a single line. And if the depth was continuous, a host of low-level creatures with a normal AC of 5 would have that pushed to 3 ... making them notably difficult for a low level party to take down. It would reduce the overall chance of hitting just 10% ... but if you consider that a first level fighter will hit AC 5 only 30% of the time, the rule is reducing combat effectiveness by one third. That is very significant!
Would players deliberately not fight so they could empower their companions? If you consider the number of situations in which a low level mage "runs out of spells" and has to just throw daggers, would it be better if they simply helped the fighter get hit less? It would be interesting to see how difficult it would be to run, and if it were worthwhile for the players.
Can you hire a guy to stand behind you and improve your AC?