Since SK started its always been easy to separate armour into 2 categories: stuff that makes you hurt stuff better and stuff that doesn't. For example, there isn't much reason to use a grey feather cowl instead of a vog cub cap when the trade is shock resist for asi med.
I would suggest that additional bonuses are made available on otherwise purely defensive gear. We already have movement speed up on the merc sets, why not make it more available? Gear like the plate sets could be made to actually help you not die when you get hit by having something like 'enemy damage decreased' which would work like the damage increase boosts, but in reverse.
There is obviously a lot more than would be needed to not get too much redundancy over all the currently useless gear, but all I'm here to suggest now is simply find other bonuses we could have.
If knights are gonna get even more bonuses, give more power to monster too. We're already too strong, even with 'useless' armors.
"We already have movement speed up on the merc sets, why not make it more available? "
How many sets have global slime bonus? Chromalisk series
How many sets have sword ASI? Only Vog
How many sets buff gun ctr? Seerus mask (chaos set has lows and negattive effects)
How many sets have bomb ctr, without hefty drawbacks? Only Volcanic Demo
Bonuses are supposed to be unique to something specific. You're also contradicting yourself; you wrote "DIFFERENTIATE" in the title, yet you want to spread the same bonus :P
"enemy damage decreased"
Thats silly, just increase the defense bars :P