Take a look at the feat Expertise. It seems to be what you want -- it represents a fighter concentrating on defense. A 10th level fighter can easily live with taking a -5 penalty to attack rolls for a +5 bonus to AC.
But no, I don't really find this to be a problem. If you do, however, you could use one of the systems that give AC increases based on level. IIRC, the Wheel of Time RPG uses one. Again IIRC, it operates similarly to saving throw bonus. For the more combat oriented classes, you get +2 AC bonus at first, then another +1 for every two levels after that (matching good saving throw progression). Someone who actually has one of the RPGs that uses this could give you a better picture of this mechanic.