Antonyms for the phrase "more nascent" can include "fully developed," "mature," "ripe," or "established." These antonyms suggest that the subject in question has already undergone significant growth and development, and is no longer in its early stages of formation or emergence. While "more nascent" implies potential and promise, its antonyms imply stability and longevity. Whereas a nascent idea may be exciting and innovative, a mature concept may be refined and proven. Ultimately, the choice of antonym depends on the context and desired emphasis, but these suggestions provide a range of options for expressing the opposite of "more nascent.