The sequel to Eclipse: The Girl Who Saved the World Spoilers ahead for that.
So our intrepid young four -- Eclipse and the siblings Comet, Star, and Aurora --arrive on Mars, and the siblings go to speak to the Wizard of Mars. Eclipse is just as glad because if you ask the Wizard a question, you have to pay the price he sets. (This has ended past civilizations.)
Another young persona (superhero) arrives. Cloud, whom the three know, and so they set out to the end of the universe and back. There are problems, but not really until they return and can't connect. This is their first warning they are in a world very different, with vastly altered history and no personas. (It is not exactly our world, but closer than theirs. ) So they cope, especially because they still have two doors to fight, and since this world has seven billion people, that plus the billion of their own does add up to eight billion.
It involves coins, their wonder at a justice system without telepathy, raking a lawn, secret powers even in that world, a space shuttle, and more. Also quarrels that may remind you of the destroyed tower in the last book.