Optimise rendering engine. Don't render an object area if it's covered by an
opaque object above it.

A GUI ;-)

More object types. Gradients, curves etc

Background definitions

Modification layers

Right-click context menus

Decent text layout. Justification, word wrapping, margins etc.

Drag-rectangle for multiple selections, creating objects etc.

Toolbox.

File/New and associated document creation dialog.

Repository for temporary storage of objects.

Add polygons and line/rect/polygon/gradient clipping into imlib2

Document resizing.