It would make sorcerer's and half-casters happy at being able to enter prestige classes (far) earlier. It would make most melee prestige classes far less reliant on feat taxes.

A scaling dodge.
Dodge Prerequisite 13 Dex.
You gain +1 dodge bonus to AC. This increases by +1 every 4 HD.
At HD 2: Gain Mobility
At HD 4:Gain Spring Attack.