There’s a lot of ways to decrease potential. Failed enhancements, opening a gem socket, attaching magic amulets, trading the item, Item Awakening, and Briquetting.
Leveling up Magnum Opus increases the size of the craft grid, which won’t necessarily increase the rarities of items you make, but allows you to create other kinds of items that require a larger sized grid instead. Also, I have no clue if you can make multiple items at once with the skill.