I wont call this a 'fix' as it doesn't fix the underlying problem, however it should in theory allow AC even on non-specific AC builds to catch up enough to be 'useful' at high level (circa normal/hard Amrath/House C, no it won't make a difference in Epic, but that is a whole other ball game where AC is concerned and needs a thorough looking at).
My proposal is increasing base AC (currently fixed at 10) in line with increases to BAB, 1 to 1.
This would give, for example:
Fighter 1: 10 (base) + BAB 1 = 11 base AC.
Fighter 20: 10 (base) + BAB 20 = 30 base AC.
Monk 1: 10 (base) + BAB 0 = 10 base AC.
Monk 20: 10 (base) + BAB 15 = 25 base AC.
This then raises the achievable/useable AC bracket for all characters, especially those focusing in melee.