Add Multiple Layers
There should be the ability to add and choose between multiple layers.
Also, after choosing to mask with a shape, it would be cool to be able to mask with the brush on top of that - to fine tune it.
The app is very intuitive, I like the way it's set up. The ability to use color backgrounds, gradient, and the shapes for masking is great. I think it needs a few more bells and whistles to differentiate it from other masking apps. Adding in patterns like the Urban Decay patterns in Tangent might be an excellent place to start.