The phrase "be the horizon" refers to being in a state of endless possibilities or opportunities. Its antonyms could be limited, confined, or restricted. When someone or something is limited, it means that their options or choices are few, which can limit their potential outcomes. Being confined means being restricted to a specific location or situation, without the freedom to explore other opportunities. Restricted means having a limit placed on an individual or object's abilities or movements, which inhibits their progress. Overall, the antonyms for "be the horizon" signify a state of confinement or limitation, which is opposite to the endless potential and opportunities of the phrase.