Excellent. I'm thinking that further Shapes support might even roll-up into the 3D aspects of the app, in that 2D vertex buffers of all-shapes-and-sizes could perhaps be supported, different shaped triangles, various meshes etc.
But yeah, in summary, glad you've heard out this idea for consideration - support for gradients (with the ability to tweak alpha) across shapes (and other aspects of the app) would be useful as heck. I suspect you'll see some very cool artwork created by users once they're available.