So, if I understand your suggestion, these armor and shield bonuses would apply + fort items - level of difficulty = applicable fortification?
I really see no problem with this. Since it doesn't really favor one class over another, just adds benefits to certain fighting styles, as far as I can see.
After all, even Barbs and TWf can use a shield when they need to do the traditional tanking method. Just like against the Hound, since dps is useless against her until the little dogs are charmed and have hurt her enough, even a Barbarian pulls out a shield and shieldblocks.
I kind of lke this idea, even though it would hurt my twf DT Vestment(PJ's I guess) wearing Pally/Monk on elite (lower his fort from 100-50= 50).