Here's my suggestion...
Basically, someone with Chainmail has an AC of 17. That's it. Agility doesn't enter into it. Only a couple of Armor types allow the Agility modifier to still modify AC.I prefer Armor to just set a base AC. So Plate Mail gives a character a 22 AC. Agility modifiers don't enter into it. I can see something like Leather Armor giving a base 12 AC that can be modified by Agility. But I don't see why the heavier armors do. Something like this:
Padded** AC 11
Leather* AC 12
Studded leather AC 13
Hide* AC 13
Scale mail AC 15
Chainmail AC 17
Banded mail AC 19
Half-plate AC 21
Full plate AC 23
* - Armor Class rating can be modified up or down by Agility bonus.
** - Armor Class rating can only be modified up by Agility bonus.
The straight AC would benefit those who have awful Agility scores. While those who had high Agility scores would receive less of a benefit. A PC with an 18 AGL and Leather Armor, for example, would get an AC of 15. Same as a guy who had an Agility of 8 and Scale Mail.
What do other feedbackers think?