Drupal 8.1.0 will be released on April 20, 2016, so the beta phase for Drupal 8.1.x is coming up soon! We have until February 24 to make additions for Drupal 8.1.0. (After that, new additions will go toward 8.2.0 instead, which is currently planned for release in October.)
Our priorities for 8.1.x improvements are an experimental Migrate user interface, automated frontend testing, and most importantly bug and usability fixes.
Migrate user interface
Migrate is in core as an experimental module in Drupal 8.0.x, but it does not provide a user interface. The next step is to add the user interface as a core experimental module as well as fix a few important bugs, such as missing test coverage for the user interface and delaying migration entity saving for safer rollbacks.
Frontend testing
As we use more and more frontend components, we need to ensure that our frontend implementations are tested as well as we already test our backend. There were some prior steps introduced in Drupal 8 towards using Behat for frontend testing, and infrastructure support is almost ready. However, there is more to do.
Bug and usability fixes
There are currently over 500 major bugs filed against Drupal 8 branches. The stability of Drupal 8 is always a top priority, so help with these major issues by helping create and test patches, as well as testing to determine if the issues are still relevant. Usability issues and contributed project blockers are also especially important.