Alright if you guys feel you can't add extra dmg power on top of everything the armour already has. In the intrest of not making players even more powerful, Here are a few other suggestions that don't buff damage, but also don't feel like set effects that would work way better for tank armour.
Bind a target (not stun), if they are out of melee distance from you.
The armour can "drag", enemies within 2 squares of you towards you
A chance to prevent opponents from healing
Berzerk will no longer reduce your armour rating while being used.
Chance to temporarily freeze prayer drain, (your prayers remain active but do not drain points)
Bleed Abilities (Blood Tendrils, Slaughter, Dismember, etc) have a chance to be dealt as a single hit as opposed to damage over time.
I get that the issue of tank armour is out of the scope of this rework, and that's a completely fair response. However with this update you guys are introducing multiple sets of tank armour into the game, one of which is fairly high level (Elder Rune), and two sets of top tier power armour. Because of this you still have to contend with the issues that we have with armour right now, so while there's no expectation to solve the problem. Don't make it worse by putting a tank effect onto power armour.
As for the degrading question, I say no. This armour will be expensive as is, and honestly everyone who uses it will quickly augment it. So no point adding extra utility to this armour, when honestly it won't be very much used in it's non augmented state.