In Legion, some armor sets will finally have additional 3D pieces, rather than the few limited shapes per piece that have been used in the past. Currently these are mostly used on the Tier 19 / PvP armor sets. These new pieces have to be modified for each race and gender, similar to helms.
Keep in mind that this is an !
The Demon Hunter Tier 19 set has an extra piece of armor on the gloves, boots, and hips.
The Death Knight Tier 19 set has an extra piece of armor on the gloves and boots.
The Mage Tier 19 set has scrolls added to the sides of the robe.
The Paladin Tier 19 set has extra plates on the hips.
The extra pieces on the Rogue Tier 19 set aren't positioned correctly yet. They go on the boots, gloves, and hips.
The Warlock Tier 19 set has an extra piece of armor on the gloves.
The Warrior Tier 19 set has spikes on the boots as well as a medallion on the chest and gloves. Currently there are only colors for the Mythic extra pieces.
Also worth mentioning is the quiver given to Marksmanship Hunters and Libram given to Holy Paladins.