Can you really imagine this happening:
Class Dev: Hey Ion, we put in this amazing concept for a new class called the Time Warden. Take a look!
Ion: Holy smokes guys, this is amazing! The mechanics are amazing! People will love the way this plays. The story is just perfect, it fits what we want exactly! And the art... Wow. The effects and animations are killer. I love it!
Class Dev: Amazing!
Ion: But... Wait... This wasn't a hero class in Warcraft 3, was it? Sorry guys, we have to shelve it. I know it would be awesome and would be the high point of an expansion, but we can't break the hard and fast rule of only adding new classes based on Warcraft 3 hero classes. I guess we'll have to make a Tinker...