Do you have the Deluxe edition? It has an additional chapter, “Arts of the Gyre”, with several partial classes that use arts for customization. I’d look there for inspiration as well as in the user-created content post on r/wwn. In particular, Kevin Crawford has posted a prototype alchemist, which I believe will be included in some form in an upcoming supplement for WWN.Do you have any thoughts on how to approach this?
It seems like partial classes choose between ten and fifteen arts (without a relationship between how many picks they get and how big their pool is). You can look at existing partial classes as benchmarks for arts, but the “Uncanny Powers and Abilities” section of “Creatures of a Far Age” chapter might also be useful. Arts seem to be equivalent to a 1 or 2 point power. If your idea for an art seems too much better or worse than that, it might need tweaking.