Move block / container templates into main templates directory, to facilitate blocks which render JavaScript to update themselves and blocks which can be both blocks and containers.
Embedding Improvements - e.g. fetch any module block via GalleryEmbed, maybe finally polish the SoC embed module. See Gallery2:Embedding:Development Roadmap
Important to improve situation with yahoo, fantastico and other re-packaged distributions that are very slow at updating or bad at providing a quality product)
Usability: Permissions Management - Add a default / easy mode and keep what we have as an expert view. Plus improvments to the existing view / controller.
Add support for site-wide status messages (technically a major theme API change, but we may find a way / trade-off to add it w/o)
Add support for per-page impression hooks / "invisible blocks"
Usability: AJAXify the progress bar
make it a DHTML overlay instead of an interstitial page
Stability: Overhaul the rewrite module (my require API change)
Change the rewrite module so that it has a minimal footprint in the .htaccess file. Just enough to delegate to the PHP code and let the PHP code interpret the url.
Allow each module to register urls that it wants to control, like Drupal does
Features: Rewrite the RSS module
It's way, way, way too complicated. Replace it with something very simple
Support views delegating to other views.
Hooks in register module to support OpenID, LDAP and HTTP server auth. (User:Jablko)