I noticed that all Median XL sets are 5 pieces which corresponds with WOW 5-piece sets (well at least until recently where they disappeared completely, replaced by some shitty AP grinds)
Thing is, WOW sets always had this 'surplus' piece you could swap out. You had your 2pc and 4pc set bonus but the fifth piece was for you to decide. You could use the full set OR get some non set piece with better stats.
That being said, wow items are pretty boring. They are just stat sticks, 99% of them don't do anything special. In this mod you could really do much if you could get the full set bonus wearing 4/5 pieces. Replacing one piece with a SU/RW, getting various different combo builds.