The title is a bit confusing, but the image below gets the idea across pretty well:
If you aren't familiar with the ensemble system, it basically rewards you with an item only if you own a set of other item skins to complete the "ensemble". If we applied this system to the costume collection, it would solve the issue of some of the costumes looking inconsistent between the paid version, and the time limited version. I imagine that it could be tedious to implement on Klei's part, but with all the assets existing, it might not be too tough.
@splorange pointed out the inconsistencies in their thread