This is a brainstorm page, raw and ready.

Graphics capabilities

cairoification
SVG 1.1 complete and performant
- solid testing of opera-interop
Canvas improvements
- OpenGL ES context?
- %age width/height
- finish Safari compatibility
3D markup for retained-mode and model manipulation?
transforms/filters for images and canvas
XUL2D
- affine transformations
- non-affine transformations (selected only?)
- transforms expressed in CSS
- declarative animation
- pan, zoom, scale
- translucent and non-rectangular windows
- text and others on paths
- flow and flex around complex shapes
3D work
- X3D?
Level of detail
- Thumbnailing
- fast-but-general for animations
- content model LOD?
downloadable fonts
vertical text
printing improvements

JS2

improved template builder
- data providers for sqlite, RDF, filesystem, computed-by-JS
- produce arbitrary content
- unprivileged use
XUL box model
- hyatt's spec and CSS properties
user-implemented layout managers (a la grid, stack, deck)
improved widgets
- tree (esp "view" API), sort for unprivileged content
- grid
- stack and event handling
- support for large-scale data containers (not just in tree)
overlays
- synthesized onload (at the least: declarative event handler merging)
- xpath insertion-point selection
- fallback locations
editing improvements
- NVU changes landed
- richer editor interface
whitespace preservation in XUL, for editor roundtripping
error reporting
- debugging/tracing
Cocoa widgets
remote XUL
- sync/async for DTD loading, etc.
- security issues
single-file apps (script, XUL, XBL, DTD, CSS)