Nut and Geb are aspects of Gaia and Ouranos from Heroes of OlympusThey have very different personalities, just as Aries may be a Blood Knight Jerkass while Mars is a reasonable Sergeant Badass. But they both have Earth and Sky being held apart by another power. Given the events of Heroes of Olympus this may have made taking a message between them a case of Nice Job Breaking It, Hero, but it also may give some hope: if they can arrange for the Geb aspect to supplant Gaia, mother earth won't be evil anymore. That would fit better with the ecological message from the previous series.
The Faust family is descended from the Faust.The universe of this series fits All Myths Are True and at least some elements of the Abrahamic religions are shown to be true (Moses defeated the magicians of the House of Life). So it's possible that Mephistopheles exists in the story's universe as well.
In-universe Lauren Faust is a wizard.She is a wizard, but never used any ancient knowledge on her works beyold the point of reference, all her doings such as the creation of My Little Pony: Friendship Is Magic are pure skill.
Related to above, in-universe brony comunity is partially aware of Lauren Faust being a wizard.This is treated mostly as a meme by the comunity and no one actually takes it serious, however, the few bronies who too are wizards have a deep respect for her.
The books' titles relate to their endings.
There will be a three-way crossover with The Kane Chronicles, the Percy Jackson series, and Rick Riordan's future series based on Norse mythology.
The Egyptian God of Ice will be discovered in the above-mentioned Massive Multiplayer Crossover.
The recordings themselves will play a role in the crossover.The second book makes it quite clear that the Kane Chronicles are being published in-universe. While the Literary Agent Hypothesis is nothing new, it would be a good way to unite the two casts, so at least one of the demigods will have read them. Nico's a likely candidate - he's nerdy enough, and he'd certainly be interested in Egyptian mythology.
Felix will be the main character of a Sequel Series