Well, pretty much everything!
Despite the many pages within this book, we have only just scratched the surface of this wonderful topic of AI in games. How many times did I say in this book "Unfortunately, this [insert topic] is outside the scope of this book"? Too many to remember. This is just for the built-in AI Framework of Unreal (for instance, we haven't covered how to properly extend Generators and Tests for EQS, or how to create a custom AI Sense). However, I strongly believe that this book gave you some solid foundation to iterate upon and keep learning the Unreal AI Framework. Hopefully, you will be able to use this book as a reference for your work in Unreal.
As much as I hope that this book has been helpful to set the first stone, the intention of this section is to indicate that the path is made of thousands...
