The term "more substantive" refers to something that is more significant, meaningful, or valuable. An antonym for this word could be "less substantial," which refers to something that is not as important or meaningful. Another antonym could be "insubstantial," which means something that is insignificant, flimsy or not substantial at all. Other suitable antonyms for "more substantive" could be "trivial," "lightweight," or "unimportant." These antonyms perfectly illustrate the opposite meaning of the original term and help to clarify that their respective meanings could not be more different from each other.