The phrase "are on horizon" implies something new, exciting, or upcoming. But, what about its antonyms? If you replace "are on horizon" with "behind the times," it could connote outdatedness, backwardness, or irrelevance. Alternatively, "losing ground" could suggest losing momentum or falling behind. "Stagnant" could imply lack of change or growth, and "fading away" could mean slowly disappearing from sight or memory. On the other hand, "in the past" could indicate that something is behind us, while "old hat" could be used to describe something that is no longer fashionable or trendy. So, depending on the context, the antonyms of "are on horizon" could signify something negative or simply signify the absence of novelty.