I have always wondered why the almighty Horoth could only offer a moderate fort. item while any level 9 or above character gets a heavy fort. item... Sure, it would make rogues and critical amenities useless, but reducing fort. against the most common logic isn't a very good solution imo.
Here is a proposition:
* Allow intelligent (perhaps humanoids only) ennemies wear fortification items according to their rank/level. They should probably get them later than characters, because characters are heroes 
* Change Sunder this way (the bold part indicates what changes):
Code:
This melee special attack, when successful, results in a -25% fortification
penalty to the target for 15 seconds if it fails a
Fortitude save (DC 10 + Str mod).
Some creatures may be immune to the sunder effect
Destruction and Imp. destruction items would keep the same effect (-4 AC both, stackable with each other).
* Modify Fighter Strategy (Sunder) this way:
Fighter Strategy (Sunder) I (1 AP): adds +1 to Sunder DC and reduces fortification by an additional 5%, for a total of 30%.
Fighter Strategy (Sunder) II (2 AP): adds +1 to Sunder DC and reduces fortification by an additional 5%, for a total of 35%.
Fighter Strategy (Sunder) III (3 AP): adds +1 to Sunder DC and reduces fortification by an additional 5%, for a total of 40%.
Fighter Strategy (Sunder) III (3 AP): adds +1 to Sunder DC and reduces fortification by an additional 5%, for a total of 45%.
* Change Opportunist feat into:
Code:
A Rogue with this ability gains a 3% chance to double strike
with melee weapons and bypasses 20% fortification.
(this reduction stacks with sunder effect)
* Change Improved Sunder into:
Code:
This melee special attack, when successful, results in a -40% Fortification
penalty to the target for 30 seconds if it fails a
Fortitude save (DC 14 + Str mod).
Some creatures may be immune to the sunder effect.
Additionaly, Raid bosses should also wear Elemental absorption items...