I think the custom rule feature should have two more layers.
You should be able to assign subtasks to team members and not just the main task. This would require just adding another layer to the “action” options after you select a trigger like “move to this project/column”. The action should allow you to assign a subtask; right now you can only assign the main task.
While you can add/tag team collaborators to a task using the custom rules, I believe there should be an option whereby two team members can be assigned the ticket at once. Sometimes two people need to work on one task/ticket at once for different reasons, but you can only assign a task to one team member at a time.
These features would be extremely helpful.