Design a method of configuration-sharing
In Drupal, migrate_plus provides a "group" functionality (max one per "task") which is used by CLI/UI tools to group migrations for presentation, and also provides common default configuration to each member of the group. In Soong, I threw in a half-baked "subtask" functionality, with the idea being that a task containing subtasks could provide the shared configuration. On second thought, that seems unnecessarily complicated (it's causing me some trouble implementing the status command) - and, really, the grouping functionality and shared configuration functionality should be separate.
So, how should tasks share configuration? Something like PHP traits (or C #include), where (if using YAML for your task configuration) you could include a separate YAML file at an arbitrary point?
This probably isn't something that belongs in the Task interfaces (which don't know whether their configuration is coming from YAML, JSON, or whatever) - it should be implemented at a higher level than that. But, at a lower level than the Symfony commands we have now - it should be a utility usable by any tools that wish to take advantage of it.