The amount of AP you have maxes out at 20. The amount you'll often have is about 6. That's too small for percentage to be a useful way to manage it.
Different weapons already have different AP costs. Heaver gear also decreases your movement per AP. I'm not sure making heavier weapons and equipment penalize you more on top of what they already do would be a good idea. Warriors do still have to move around the battlefield and attack enemies.
Warriors, on the front lines, will tend to absorb the first poison/fire damage, and you want them to suffer even more penalties to AP. Either the penalties you'd apply are so small as to be no different than the status quo, or they'll make being a warrior a frustrating experience in sitting around doing nothing.
I assume that your idea of having static AP/AP regeneration means your attributes don't count for AP. That just seems to disproportionately punish melee/tank classes.
