Have Subtasks have their own separate Custom Fields (e.g. parent task has CF x,y,z while subtasks don't have those custom fields but instead have none or their own applied custom fields).
This would be a major improvement for usecases like projects:
  • The project (parent task) holds all the high level project information
  • Subtasks of a project are clear tasks/deliverables for assignees.
Currently each subtask of a project is bloated with empty high level custom fields from the parent task.