It's an interesting question. I will give us credit as humans to come up with a “social contract” (Hobbes) and extensive laws (Hammurabi). However, I don’t see how we could agree on ideas such as “Love your Neighbor” as a legal imperative, and various subtleties of ethics and details of laws without a Torah from a Creator. For example, the Shabbat and the way to observe it is one of the most revolutionary ideas of Judaism, and not something any person or nation introduced beforehand.
One of the very early scholars, Rabbi Saadiah Gaon, points out that even if we could come up with the prohibitions against theft and murder, the parameters and details of those prohibitions are something for which we need revelation. Even the idea of the sanctity of life is not a concept of the secular, logical mind but a revealed concept. Some (for example, Kant) would argue that even the very concept of moral imperatives, "I ought to" is a revealed religious concept, and not something that has any place in a human created system of laws.
So, I would say there is very much – if not virtually everything – that we need the Torah to teach us.
