I never thought I'd say this, but I think Sylla's not wrong on this one. Additional stances might be difficult, and maybe ultimately not worth the time, but it would still be very fun. Think of the good old scotish "enormous sword held over the head" for instance: You can get good head blows in very easy, but parrying would be much more difficult. Best make it a short fight! It would add even more variety to the NPCs you end up facing in the arena (for instance), and a lot more variety in the characters you can make.
As far as mechanics are concerned, I'd see it not so much as a skill but something you acquire independently. Maybe you just pick one at character creation and that's it for that toon, or you get new ones when you raise close combat skills (in addition to the combat skill itself of course). I don't think they should be made so that one is inherently "better" than the other, just more appropriate for your individual fighting style.
I honestly don't think it would be very difficult to code (I of course say that in a purely speculative manner, not being familiar with the code). If you think about it, every weapon class has it's own "stance" as is. Your character doesn't hold and swing a quarterstaff like he does a hammer or like he does a poleaxe. These guys spent 4 YEARS on their engine, I suspect somewhere along the way, it occurred to them to create a system wherein it's not that hard to generate new movements (provided you're familiar with the engine of course).
Anyhow, I'd like to see this happen personally, somewhere between the NPCs, the MP, the replays and the rest of my ridiculous and unrealistic wishlist XD