Armor Set Bonuses
I think it would be really cool to add a component to dungeon/raid sets where once you acquire the full set, that set's bonuses are "unlocked" and then can be swapped out like REs.
As an example, let's say the Tier 4 set bonuses are best for my build...but Tier 5 is the current content. Because of the stats on the higher ilvl gear I'd have better performance output with the Tier 5 armor set BUT I have to alter my build to be effective in raids in order to take advantage of the Tier 5 set bonuses. This forces players to take certain builds over others based on the strongest gear that is available. Instead, once I acquire the full Tier 5 set I'm now able to "swap" set bonuses. So now I get the item stats from the Tier 5 set BUT I'm using Tier 4 set bonuses.