Make it a wondrous item that occupies the shield slot. Otherwise you get a stacking benefit. Since miss chances aren't 'aligned' with the shield slot, it'll still be a slotless item that costs double, and worth it for casters since it lets them use a slot they normally wouldn't.
Alternatively, have it require shield proficiency to actually 'set in motion'.
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THW weapon dmg is most accurately reflected by the weapon itself. The problem with letting huge amounts of Power Attack dmg accrue to THW is rapidly power attack overcomes the importance of the weapon itself.
THW should: Do higher base dmg then one handed weapons (they do) and let you deliver all your strength in one blow (they do). It's also the easiest style to teach someone.
4E got this one almost exactly right. They probably should have made 2hW a little bigger, but since they locked Shield benefits in at +2 AC, +2 Reflex, it's all good. They also locked wielding non-ThW used in two hands at +1 dmg...also good in their system. A longsword in two hands shouldn't be as much dmg as a Greatsword in 2 hands.
Consider: A greatsword in the hands of a Fighter with 18 Str vs a longsword is 2-12 +6 vs 1-8 +4, or 13 vs 8.5. That's a dmg bonus of +4.5, and it only gets HIGHER as Strength goes up. At a Str of 30, the difference is now 2-12+15 vs 1-8+10, or +7.5.
+7.5 dmg JUST for changing your weapon style is HUGE. Giving THW twice as much Power Attack? I'm sorry, but Power Attack favoring THW is what breaks combat styles now. Let THW get it's extra dmg from strength. There's no reason it deserves up to potentially +20 more dmg, as the current system allows it. The mundane dmg difference should be reflected by Str and a bigger weapon.
(Besides, it'll also stop all the nonsense about people trying to charge with a lance in two hands for full Str + 2x PA...)
As for 'reality', I would like to point out that Dual Wielding is historically practiced only in dueling situations and in circumstances where armor is not allowed. All the fancy, schmancy dual wielding moves tend to be irrelevant against a good suit of armor. If you needed a second weapon, you generally employed your shield...it was big, hit hard, seldom missed, and protected you. Trying to do the same thing with a main-gauche took considerably more training and skill, and was not something you generally did on a battlefield. Only the advent of gunpowder weapons that made armor and shields irrelevant actually let TWF evolve into something it was good to know, because against armor the style generally sucked (hence, -2 th). On a battlefield, the only reason you generally pulled a typical 'off-hand' weapon against someone in armor was when they were already down and you wanted to shove it in their eye slit.
Heck, if we look at our favorite overblown dual wielders, Drizzt and Entreri, note that neither of them actually wear substantial armor, nor do they really ever fight someone with it.
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Thus, I am SOLIDLY in the camp of Power Attack being the same for all styles. Furthermore, so as not to discriminate against THW, I'd restrict the benefits thereof to Primary Hand...this way THW can't complain that SAB Bashers and TWF fricassee machines get 'twice the benefit' (conversely, you could seperate the benefits into 1/2 per weapon, instead).
Improved Power Attack should go to 3:2, Supreme Power Attack 2:1. Each 'upgrade' is instantly worth 2-10 pts of dmg, if taken any time after level 4, and that is a SUBSTANTIAL dmg kicker for a feat...and it scales!
(Note: I still like the idea of +1 dmg for every 1 you exceed hitting someone's AC by as the 'power attack' default instead, and increases in TH 'scaling' the dmg by level.)
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TWF, just have one feat for proficiency to get rid of the penalties, and a second feat to allow all permitted iteratives.
I'd probably specifically forbid allowing Shield Bashers to get more then one attack with a Shield. This keeps Shields from a) ever being used as a primary weapon and b) keeps the two weapon advantage to pure weapons and TWF. Shields were never designed as primary weapons, and shouldn't be allowed to do the full job of secondary weapons (they're the best off hand weapons you can get). Keep the differences between the styles plain. Remember that even Captain America hits stuff with his fists and feet more then with his shield.
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The reach advantage of weapons spurring AoO is the big benefit of a bigger weapon. Greatswords are hard to ply in tight quarters. Against a trained combatant, you get the whole Jenkin symbology going, where if your first thrust/cut with the Greatsword is parried, you leave yourself totally wide open to infighting. It's easier to just hand-wave away the benefit at 'partial reach' vs the penalty at 'close quarters' then it is to assign a full benefit.
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I'll note that if you give AoO/Combat Reflexes away as a free feat, you open the door to using AoO as a mechanic to generate swift/immediate actions. This is a Good Thing.
Robby, I don't remember seeing anything on how you were going to rebalance armor, wearing Heavy Armor that was actually BETTER then wearing Light armor, all things told. Did you come to a choice on that? The 4E version ramped heavy armor all the way up to +13, +1 Dex for a total possible benefit of +14. Light armor maxed out at +10 (0/+10 dex, +2, +8 Dex IIRC), with each category of armor improving max limits by +1.
1E and 2E did it by simply waiving Dex limits for all armor, leading to absolute AC advantage in heavy armor, but they had considerably more stat limitations, too, which curtailed high dex/high armor stat abuse.
Das Bier!
