The problem with adding too much is it essentially becomes a game on it's own. The structure I have added allows others to learn from how I've designed and layed out various elements, and then build upon it.
The one enemy shows how to set up enemies, so it can be replicated and reproduced for others. If I add too many enemies, your game is not unique, as everyone ends up with the same. I really wanted the engine to be a base demonstrating best practices, allowing you to extend with the knowledge from the examples in the project.
In saying that, I'll look at adding a few enemies that I had used in my Tile Based Platformer Udemy Course, and I guess people can just not use them if they don't want to.
Thanks for the feedback though, and also thank you for purchasing.