The word "predesigned" refers to something that has already been designed or planned beforehand. Its antonym would be "undesigned" or "unplanned," suggesting a lack of predetermined structure or preconceived notion. Other antonyms could include "spontaneous," "improvised," "ad-libbed," or "unscripted," conveying the idea of something that's created on the spot. Alternatively, "custom-designed," "tailor-made," or "individualized" could also be considered antonyms, indicating that something has been created specifically for a particular purpose or need. Overall, the antonyms for "predesigned" emphasize unpredictability, originality, and uniqueness, providing a useful counterbalance to the rigidity and predictability of pre-planning.