So what does allow for more attacks? Well, anyone who has trained for actual hand-to-hand combat knows this: the reduction of a deterrent threat. In other words, I can attack more when I no longer have to watch out for my opponent's counters. In practice or in the ring, ceasing to threaten a counter attack can swiftly draw a flurry of strikes.
Alright, but that's what AoOs do now: You take your guard down, the other guy gets an attack. Now if you were to want to model reality better, you would have to say that....
ok, i'm jumping on an entirely different train of thought, here: Actually, Armor Class! Why do you get all sorts of AC, but not DEFENSE AC? Because really, that's the most important kind in a lot of melee combat.
So basically, what should happen is that you get your BAB, or a fraction of it, to AC whenever you actually threaten the square where the attack is coming from, and you don't get it when you don't threaten, in addition to provoking AoOs. Now this creates two problems:
Reach weapons now just got better, yet.
Ranged weapons never have to deal with being threatened.
I'm thinking this is too big a change to quickly incorporate into any set of house rules, but... short of straight up increasing all AC by BAB in melee, and thereby making melee combat very defense-heavy, how could such a thing be designed? I must say it gets very hard in straight up single-roll mechanics, whereas parrying by roll mechanics are tedious. Also, it's not really the parry itself that's working here, it's the threat of a counter-attack.
So the elements of attack are
BAB + bonuses + d20
The elements of defense are
10 (for the d20) + AC + bonuses
For an ideal 50% hit probability case, AC + bonuses are equal to BAB + bonuses.
Now strangely enough this pits a straight-up scaling mechanism against a monetarily scaling mechanism.
But how about this: Using the AC as DR optional rule from Unearthed, all AC bonuses from armour are halfed, and I believe excluding the magical bonuses, but we would probably have to include those.
Now:
This could make the total AC:
10+1/2 armour bonus + dex + shield + other/magical + BAB!
And you gain 1/2 armor as DR/-
Total attack bonus
1d20 + BAB + Str/Dex + enhancement + other.
So...
For example, at level 1, wearing full-plate:
AC = 10+4(Armor)+1(dex)+1(bab) = 16; DR 4/-
AB:
1d20+4(Str)+1(Bab) = 5 (15.5 average)
At level 10:
AC = 10 +5 (armor+2) +1(dex) +10(bab) = 26, 25 w/o dex, 16 AoO, 15ff, but 21 touch!; DR 5/-
compared to regular:
10 + 10(armor) +1(dex) =21. Here you can see how useless heavy armour really is. In my game if you want to have reasonably good AC you will want 30 by level 10.
So now you have a system where shields are more valuable because they won't get reduce from provoking, total AC will probably be higher, and combat is more ablative. Also, hitting with touch attacks will get pretty difficult, eventually.
But it goes one:
When making power-attacks, what you're really doing is not making a wild powerful attack, but rather one that will leave you more open after the fact. So, we can easily make Shocktrooper's one maneuver obsolete by simply saying that Power-attack ALWAYS goes off BAB-AC, and not attack bonus.