So hear me out. Imagine 1 slot for the extoic and 4 legendaries to match the set once built utilizing piece by piece additional stats and bonuses and an awesome ability or effect that only a full set can do. Kinda like how Diablo has armor sets with added powers, etc. Start of 3 sets per sub class.
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These sets it should be like Void set: 1 piece: void damage is increased by 4% 2 piece: void buffs last 3 seconds longer 3 piece: targets killed with the volatile debuff explode and apply a weaken nearby targets. 4 piece: increase weaken debuff by 10%. 5 piece: always have volatile rounds. Exotic armors that affect a subclass count as a freeby for things like void sets.