While major improvements were implemented towards user gameplanning and playcalling, custom playbook design and function continues to receive little attention. The powers that be at EA are much more focused on marketing "new features", rather than fully fleshing out those already in existence.
I would like to see custom playbooks start off as a skeleton. From there, the user should then be able to select the formations and plays they want to add, have them stack in the order they want them to appear and be able to fully utilize the formation subs feature. I think this will eventually happen, but it certainly doesn't appear to be a priority.