Stage Builder, do you get more parts?
There's nothing more to unlock via the stage builder. With any luck, Nintendo will add content in subsequent updates.
Unfortunately, so far it seems like you can't. The stage builder seemed like such a cool idea, but the fact that drawing anything precise is impossible, it's incredibly difficult to place grabbable edges, and lack of obstacles/moving parts kind of let me down. I'm sure someone more creative than I can utilize it better, but I really hope they release an update with fixes for edge grabbing, more parts, and decorations or something.