I think it was heavily confusing that "The Conflict" was created in Ideas and Concepts at first, though I can understand why; the forums were vaguely new, and new members probably had no idea that the conflict in a game would be apart of the storytelling. It should really be moved over here by one of the admins when they can.
To answer your question, the Design subforum should definitely be primarily used for game concepts, features, ideas, like how the player can interact with the world. Not about the types of enemies that they'll have to deal with. Any art, as well, would be kept in the Art subforum.