This revisits and rebalances the various feats that make up the two-weapon fighting style, bringing the feat investment needed for effective two-weapon fighting more in line with the feat investment needed for effective two-hander fighting.
Note: Condensing feats in this manner requires reworking the ranger bonus feat progression for dual-weapon fighting. I recommend you take a look at my swift hunter class (a ranger remix) for ideas on how this might look.
DUAL STRIKE [Fighter, General]
You are an expert skirmisher skilled at fighting with two weapons. Your extensive training with two weapons allows you to attack with both while moving through a chaotic combat or fighting a running battle.
Prerequisites: Two-Weapon Fighting, base attack bonus +6 Benefit: As a standard action, you can make a melee attack with your primary weapon and an attack with your off-hand weapon. You take the normal penalties for attacking with two weapons. Advancement: If you also have the Combat Reflexes feat, then you may choose to attack once with both weapons when making an attack of opportunity (incurring the normal penalties for attacking with two weapons).
TWO-WEAPON DEFENSE [Fighter, General]
Your two-weapon fighting style bolsters your defense as well as your offense.
Prerequisites: Dex 15, Two-Weapon Fighting. Benefit: When wielding a double weapon or two weapons (not including natural weapons or unarmed strikes), you gain a +1 shield bonus to your AC. Advancement: This bonus increases by +1 for each 5 base attack bonus you have above 1 (to +2 when you reach a +6 base attack bonus, +3 when you reach a +11 base attack bonus, and so on). When you are fighting defensively or using the total defense action, double the bonus.
TWO-WEAPON FIGHTING [Fighter, General]
You fight effectively with a weapon in each hand, and you can make extra attacks each round with the second weapon.
Prerequisite: Dex 15 Benefit: Your penalties on attack rolls for fighting with two weapons are reduced. The penalty for your primary hand lessens by two and the penalty for your off hand lessens by 6.
You gain an additional attack at your highest base attack bonus with the offhand weapon. You gain additional attacks with the offhand weapon at progressively lower attack bonuses whenever your base attack bonus grants you additional attacks with your primary weapon. Advancement: If you have the Weapon Focus feat for each of your weapons and a base attack bonus of +5 or greater, you may reduce the penalty for two-weapon fighting with a one-handed weapon in your offhand to -3/-3. When you reach base attack bonus +9 or greater, reduce the penalty to -2/-2.
This is good. It requires you to only take one feat to have double the number of attacks, rather than 3 feats. It makes two weapon defense a good option as well.
I'm confused on one thing, the weapon focus and base attack bonus thing says it reduces a one-handed weapon's penalty to -1, which seems like too big a penalty reduction, and it doesn't say anything about light weapons.
The advancement option you're looking at reduces the penalty for using a one-handed weapon (instead of a light weapon) in your off hand. This is essentially the Oversized Two-Weapon Fighting feat, phased in one piece at a time.
The idea behind the advancement ability is that the Oversized Two-Weapon Fighting Feat is removed from the game, and instead, you gain the benefit of that feat by having Two-Weapon Fighting, Weapon Focus, and base attack +9.