I hope blizzard has so forethought on sets.
IF sets are back, and if they are more than cosmetic i.e set bonuses that effect damage then Set bonuses must ONLY WORK inside raid content. (much like the trinket set from Nya’Lotha).
If set bonuses work outside of raids then it should be purchasable with a currency and not be a flat item drop. The best way to make sets attainable is via the weekly chest. Given the weekly chest has loot based on what you’ve done for the week it should reward currency (similar to titan residuum) for set loot based on the hardest stuff you did that week.
It should not be a situation where all mythic raiders have their tier set and dominate all content due to set bonuses simply because they do raid content.
This would also help raiders and stop taking up a loot drop slot for set items that most hardcore raiders have within a month.