To get CKEditor 5 ready in time for Drupal 10, we need to get it to at least beta stability.
See the roadmap at: https://www.drupal.org/project/ckeditor5/issues/3201824
This isn't the only work to be done, however, there are lots of CKEditor 5 plugins which could be provided by a Drupal project, there are many CKEditor 4 contrib projects which could be upgraded.
see: https://www.drupal.org/project/ckeditor5/issues/3207845
The main module and how to add a contrib module both need more documentation as well,
see: https://www.drupal.org/project/ckeditor5/issues/3205654