The phrase "leave flat" implies something that is left without any modification, alteration or change. In this context, the antonyms would be words that connote action, transformation or variation. For instance, "embellish," "enhance," "develop," "improve," "evolve," "upgrade," "transform," and "amend" all describe the opposite of "leave flat." These words convey that something has changed in a positive or negative way, or that it has been enhanced or improved. Therefore, if you want to express that something has undergone a significant change or transformation, it is best to use one of these antonyms instead of "leave flat.