I have to disagree with that Mantis. Sure, CGI can allow you to create more things but you're still limited by reality. It has to look like it blends into the real world so the designs have to be done in a way that would look like it could potentially exist in our world. You'll hear a lot of filmmakers talking about how they want their movie to feel real and grounded in reality so, even if we can go for something otherworldly, it still has to conform to what our reality looks like, at least somewhat. Compare the mechs from District 9 to something like, say, Gundam or Gurren Lagann. Anime (Or animation for that matter) doesn't have this problem so you can create things that are much more stylized and abstract compared to a live action setting so it allows the imagination to go much wilder.