I don't want to say that you have to keep your ideas in your mind. What I am trying to say is that you should try take make a top -> down (or the other way) discussion.
Start at a high level and go into detail (or start at a low level and go to high level).
For example:
- herding is a cool feature
- how do you think is the behaviour of the herd
- what animals are in a herd
- remove/add animals to the herd (your point)
- combine herds
- max size of a herd
- movement of a herd
- etc.
So you see there are a lot of thing to think about and one of them is your point. I am more the development guy and so I have to think about the whole concept of something. To add specific attributes in modern programming languages is not that big deal as long as the architecture is nice