(Stretching the definition of the "core" group a bit here, but this is important to Drupal 8, so hopefully this is ok. :))
Now that Drupal 8.0.0 is nearing its final release, the next task in front of the Drupal community is porting ALL the contributed projects! Luckily, this effort is very much underway, but the faster the majority of big modules are at least usable (ideally with stable releases), the faster Drupal 8 adoption will take off.
After talking to numerous project maintainers, including those with multiple Drupal 8 core commits, it seems like many would find value in having dedicated times during which to collaborate with other people porting projects to D8, get questions answered, get advice on sticky problems, and figure out where best to help.
If you'd like to help mentor these sorts of office hours, please add your name to the issue summary at http://www.drupal.org/node/2612094 and fill in the Doodle.
Rock!