Benefit: For every Two Weapon Fighting feat or Combat Mastery melee ability your character has, gain +1 Shield bonus when wielding two weapons and not using a shield.
Why: It scales with your level, making it useful your entire career. It helps the TW Fighter and Ranger to catch up in survivability, as they are severely lagging in AC, without stealing the Sword and Board's advantages. It's a solution to a problem that the Two Weapon Defense chain tries to resolve, but can't because, rangers are feat-starved. It provides an incentive for Melee Rangers not to multiclass into Dervish at the earliest opportunity.
Source: My DM proposed it to me in his 3.5 Age of Worms campaign.
Previously misposted in the classes and races forum...
I think that modifying the Two-Weapon Defense Feat to automatically upgrade when you gain further feats of the Two-Weapon tree (and so eliminating the need for the improved and greater feats) would also solve this...