The speaker starts with a big, open wish: to be anything at all. But the first thing they choose isn't glamorous. They'd be the thing that "eats you up inside," or the person you dream about and can never have. That tells us the speaker wants to haunt someone, to be unforgettable even if it hurts. The wish sounds less like ambition and more like longing for attention.
The second half turns to doing. The speaker would do what others called impossible, go where "you" have never gone, and say what no one has said. The repeated "I'd" gives it a determined, almost defiant rhythm. Then the last line lands with a twist: if the speaker ran the world, "you wouldn't be." It's ambiguous. It could be a threat, a bitter joke, or a sad admission that the person would vanish from the speaker's world. The mood is wistful, a little resentful, and quietly vulnerable.