The speaker is addressing someone who once doubted them, or perhaps someone who once boasted of big plans and never followed through. The first half is a list of bold wishes: to haunt this person's thoughts, to do what they said couldn't be done, to go where they've never gone and say what no one has said. It sounds confident, almost defiant.
Then the tone turns cold. If the speaker ran the world, this person wouldn't, and would shrink to a "face in the crowd." The repeated words "unremarkable," "unrecognizable," and "indistinguishable" pile up like insults. The image of someone in the nosebleed seats, waving and screaming without being noticed, is both pitiful and a little cruel.
The ending shifts again. The speaker remembers when this person claimed they would run the world themselves. That last line suggests the speaker's feelings are tangled up with disappointment, rivalry, and maybe lingering hurt. The mood is bitter, proud, and quietly stung.