Drupal 8 = Drupal 7 contrib
Drupal initiative work is spinning up. Research, planning and scoping things. Important, but up till a point. Many of the changes we want to bring to core for Drupal 8 can be done as contributed projects right now.
Starting in contrib has some important advantages. You get to ship your project earlier and more often. Drupal 7 sites can benefit from your solution. The code and concept can be tested and improved based on feedback coming from actual use in the wild.
Can't speak on how things work best for refactoring underlying framework plumbing, I imagine this works especially well for new features for the Drupal core application.
Want to improve the core book module for the Snowman use case? Start in contrib, now.
Want to make Seven admin theme ready for small screens? Start in contrib, now.
Want to make better filters for content and other admin listings? You get the point…
Don't wait for momentum in core development. Develop the concept in contrib so that we can add mature and tested features to core.