When comparing Drupal to other content management systems, a major feature request is to have some sort of inline commenting tool. This is useful when collaborating with other content editors as part of a moderation workflow.
I was thinking of ways to improve the moderation experience in Drupal 8, and decided to create Moderation Note, a tool for adding inline comments to moderated entities. The features and user experience were very similar to the commenting experience in Google Docs - you could select text, add a comment, reply to that comment, and resolve/delete existing comments. I also worked on an email integration, so everyone involved in the notation process would get pinged when comments were added.
The biggest challenge here was making a user experience that felt modern and clean while using Drupal's traditional form and AJAX API. To make that process easier for myself, I made the entity powering Moderation Note restricted - it was not fieldable, had no revisions, and was not translatable. By restricting how extensible the module is I was able to confidently create a user interface that mirrored Google Docs as close as possible.
Project page: https://www.drupal.org/project/moderation_note