Well, there's actually two questions being asked here.
1. How do we make a "Disarm" mechanic that is useful situationally, but not easily abusable?
2. How do we, in general, make it possible for players to come up with creative things to do in combat (hereafter referred to as Improvised Combat Actions, or ICAs) in a fair way?
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Now, for question (1), it probably is possible to come up with a disarm mechanic that would be useful in certain situations without turning every combat into a game of keep-away with weapons, but it's a lot harder than it looks. Examples of issues you would have to deal with:
- Is "disarm" a separate power, or is "disarming" something you do with existing powers, like a Mage Hand or basic attack? Exactly what can you disarm with?
- How do we calculate the damage that a disarmed monster does with an unarmed attack? This information is obviously not in the stat block.
- If monsters can have backup weapons, do they? Do all of them? Which monsters have these? (And be prepared for your players to ask about what backup weapons they can see every combat.)
- If monsters can pick up weapons off of fallen enemies, are we going to keep track of the position of every fallen enemy weapon on the ground? What if the Wizard tries to Thunderwave the weapons away? Will that work?
Again, I'm not saying it's impossible to come up with a solution, just that it would require a lot of work.
This thread contains valuable discussion on the issues involved.
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However, it seems like the question you're really interested in is question (2), and coming up with good disarm mechanics doesn't really help solve that problem. Once you've established disarm mechanics, they no longer require "creativity," they're just another rule system players can use to achieve their goals. (Of course, people could still come up with builds and strategies to use those new rules to maximum effect. But this is not qualitatively different than what they do with existing rule systems, and is probably not the kind of "creativity" you are looking for.)
The main issue is what to do when someone comes up with a new kind of ICA, like say, throwing a rope at someone's feet in order to trip him. You don't want to just say "No, you can't do that," and you don't necessarily mind giving him an advantage in that encounter. But at the same time, you don't want to give someone a potentially overpowered power that they can use to break future battles. And it's not really reasonable to expect that every time a player comes up with an ICA, that you can on-the-spot analyze it for how powerful it would be to give him that power that he can use every time.
The main issue here is repeatability. If the ICA is something that they could only do once (like something that involves a unique feature of the room that particular battle is being held in) then you can rule it however you want, and if that leads to an easy victory then, well, you've successfully rewarded creativity without hurting any future battles. But if it's something they could do every time (like disarming), then you might want to have a way to allow them to do it once if it makes sense, but not commit to allowing them to do the same thing every time.
There have been proposals on the boards to allow you to do that. Most of them involve some sort of "stunt system" where when the player does an ICA, he himself decides how it will work (what its effects are, what checks are required, how much damage it will do, etc.) and the difficulty of the relevant rolls is determined by how powerful the effect is. This gives the payer more control while not allowing power to get out of hand. And there's usually some limit on how often stunts can be done, or some cost like APs or healing surges, so they're not doing it every attack.