We have a lot of item sets that are barely used because of nerfs or the current meta regarding the resistances system.
The main idea is that additionaly to existing stats we add some crazy bonuses when a condition is fullfilled.

for example: chaos set has a big downside having all resistances negative, for the minor bonus od increased damage- so the idea for the set is: "for each status effect aflicting the user, he gains (defense increase or weapon damage, movement speed, health regeneration- something to make it balanced)"
the user of this set would depending on the situation want to be debuffed- it still will be a risky thing to do.

other effects ideas of mine are, damage reflection, recieving a type of damage is converted into healing, lifesteal, player duplication(think legend of zelda 4 sword adventures), on death revived for X seconds,...

the conditions for recievend such effects could be : having % of life left(survivor-having half pip left/untouchable-at full health), using a type of weapon, being affected by a status effect, number of players in the party(lonewolf/partyanimal), having a full set, being in a stratum type.

By using these aditional stast i hope that the now forgotten sets will become more usefull in the future, while at the same time not decreasing the effect of already usefull ones.