God promises to punish Molech, the god of the Ammonites, and those who worship it, but the Ammonites will be restored afterward, as they are relatives of the Israelites, descended from Lot. God will also punish the Edomites, who had a marauding culture, but their children and widows He will keep alive. Other nations also have judgments against them.
Babylon was used by God to exact judgment on Israel, but now that the judgment has been exacted, and the nations became drunk on the things that came from Babylon (which would be repeated in Revelation, chapters 17 and 18), and since Babylon does not have any good left in it, God will destroy it.
Jeremiah tells Seraiah to deliver a message that he had written down some time before, a message that contains all the things that would befall Babylon, and then to throw it into the Euphrates, to sink as Babylon would sink.
The Book of Jeremiah reminds us that we must be careful to listen to what God is saying, and to be prepared to act on it.