Summarization of Nathan's suggestions:
1. Improve connector & layout tools (required_before_further_SMIL_development/biggest_result_for_least_effort) Question: Does this need to be done, independent of adding SMIL functionality?
2. Rewrite rendering pipeline Question: Does this need to be done, independent of adding SMIL functionality?
3. Determine if there is an appropriate existing timeline-based animation tool to incorporate code More details here: http://wiki.inkscape.org/wiki/index.php/Animation-(Timeline)#User_interface_...
4. Develop animation feature list (required_before_GUI design/requires_SMIL_spec_review) More details here: http://wiki.inkscape.org/wiki/index.php/Animation-(Timeline)#User_interface_...
5. Develop detailed, consistent GUI design More details here: http://wiki.inkscape.org/wiki/index.php/Animation-(Timeline)#User_interface_...