Sooner

• Hit Test graphic items and add mouse event handlers to them, for simple and powerful interactivity.
• Simple user interfaces that allow your users to tweak values in your script.
• Path intersections.
• SVG importing / exporting.

Later

• Parametrizable path offsetting / stroking, to expand strokes to outlines and optionally produce all kinds of expressive strokes easily.
• Boolean geometric path operations, such as union, intersection, exclusion, etc.
• Selectors similar to CSS for matching items in your Paper.js document.
• Typography using SVG fonts, with support for advanced typographic features such as type inside graphic shapes and along paths, with control over styles of sub-ranges of text.
• JSON data format for persistence and direct importing from Adobe Illustrator through Scriptographer.
• PDF importing / exporting.
• Running Paper.js on the server using Node.js.
• Running Paper.js apps stand-alone through a native, hardware accelerated Runtime that embeds a JavaScript engine.