Weird idea, just off the top of my head...

Sheilds, unlike armour, is a very active defense. How about getting a percentage of the character's BAB as the AC bonus for shields. For example: A 1st level fighter gets +1 AC if he uses a shield. A 10th level fighter gets +10 if he uses a large shield, +8 for a medium, +6 for a small (numbers are just pulled off of my head here for example purposes.)

As I said, weird idea. Haven't put more than a minute into working it out, because I'm in a meeting right now that I am technically supposed to be paying attention to.