Nine is an active project to build the next version of Buildbot. It is a substantial refactoring with the aim of fixing systemic issues in older versions of Buildbot, and preparing the framework for the future of CI. For more information on the goals and approach, see the release notes - https://github.com/buildbot/buildbot/blob/master/master/docs/relnotes/index.rst
Buildbot-0.9.0 will be the next release after 0.8.9. While there may be work on and even releases from the Eight branch, changes made on that branch will _not_ be merged forward to the Nine branch after the 0.8.9 release. If you're hacking on Buildbot, you're going to want to hack on nine.
Work on nine is in the master branch at https://github.com/buildbot/buildbot/tree/master.
If you're new to nine, but want to pitch in, these bugs are a good place to start. They're not necessarily "easy", but what needs to be done is pretty clear, and you will only need to study one or two parts of Buildbot to create a patch.
While most nine development aims to refactor and redesign without breaking things, sometimes that doesn't work. We'll need to fix this stuff before the 0.9.0 release.
More Substantial Bugs
These are some of the remaining enhancements to accomplish before reaching the 0.9.0 release. They are more complex than the "starter bugs" above, but are at least reasonably well-defined.
These bugs are "big ideas" that are still missing a lot of details. If you're motivated, and have ideas about how to achieve these, have at it!