Gotta avoid tying set bonuses to any one type of item as a keystone or you end up with the exact same "locked in" effect that you're trying to avoid by creating a flexible system.

What you're basically asking for another "augment" slot on everything that can be slotted with set bonus "tags" to apply a set to any group of items you wear; eg. stuff 3 things with SHOCK tags and you get the SHOCK set bonus. Slot another 5 with AWE tags and you get that bonus for a combined SHOCK & AWE effect.

You'll also have to live with people poking fun because everyone knows SHOCK is better as a 5pc than AWE.

BTW, considering that set bonuses are "artifact" type and the number of lines in them, you might as well just create individual tags to grant artifact bonus lines identical to the lines from current sets and permit them to be slotted into any named item at one per item. You'd end up with no more lines than given by 3pc + 5pc RL sets and the lines themselves wouldn't stack with existing set bonuses. It would still be a power creep due to flexibility, but you'd definitely see items being used for their own stats instead of their set bonus unlock.