Since prize boxes seem to be.... well.... the most common "content" output of OOO why not create a line that you can end anytime you want. The past prize box themes have been Status and now it is Birthstones. The problem with this is that once players get fed up with this you can't really quit either because others will complain about you not getting to the box they wanted.
Metal Prize boxes would be different, you could make any number of boxes but stop anytime you want (unless you tell us what metals you plan on using, but I doubt you would do that since you never tell us much of anything).
Gold
Copper
Silver
Steel
Rust
Bronze
etc.
But probably the most important thing I would like to emphasize is making these costumes (and future costumes) texturally unique. The current Birthstone line disappointed many people because they lacked a Gem-like sheen. Make future costumes have more than just different colors but different textures (like the Serene Helms).