Exactly, you can’t start inventing new anatomy, or if you do, you first have to have mastered real anatomy, understand in detail how it works, and your new anatomy invention has to be very well planned out, because the musculoskeletal system is not arbitrary lumps and bumps on the skin, but a very complex integrated system in which everything fills a specific role.
There is already a huge amount of room for manoeuvre within the restraints of technical accuracy, and so there is no need to go outside them. Take a look at every anatomically correct figure ever made, and you’ll see, that although they’re all equally restrained to anatomical accuracy, they’re all quite different.