WMG: Rick Riordan

    Guesses About The Upcoming Magnus Chase and the Gods of Asgard 
The villains will be the Jotuns, with Surtr as the Big Bad
Of all the figures in Norse mythology, Surtr is one of the few who is both uniformly BAD and not a Sealed Evil in a Can. The story will take pains to show the differences between the Jotuns and the Greek giants.

Loki will be be an ally of the heroes... but not the gods
He's too obvious to be the villain. Instead, Loki will be a third faction, opposing both the gods and the giants. But rather than opposing the heroes, he will try to get them on his side. There will probably be a time (perhaps one book) where the heroes are largely working for Loki.
  • Or, alternately, he'll be the Red Herring villain of the first book, similar to Hades and Set.

The heroes will set up their base of operations in the Bronx
The Greeks are already in Manhattan and Long Island, and the Egyptians are in Brooklyn. That leaves the Bronx, Queens, and Staten Island. Queens is adjacent to Brooklyn and between Manhattan and Long Island, so it's unlikely. And Staten Island is something of a Butt Monkey.
  • Jossed; the Norse will be based in Boston

there will be a comment about the Marvel Thor and Loki
It will go along the lines of 'I don't look like that, but the movies have empowered us beyond compare'
  • Well, the books are being published by Disney Hyperion, and Disney produces the Marvel movies, so that would make it even better.

Rick Riordan will follow up on the Norse mythology series with a series on Mesopotamian Mythology
New York is quite literally the Land of Two Rivers - The Hudson River and the East River, both mentioned in the final Percy Jackson book. And a Troper can dream, can he not?

The series will be followed up by a major crossover event

The "Curse of the Bambino" was a literal curse
Stemming from a dispute between the Greek and Norse gods over Babe Ruth transferring to the Yankees.
  • Following on from this, a character will have a Red Sox cap that turns them invisible (hey, there's a helm of invisibility in Norse myth too, isn't there). If said character meets Annabeth, they won't get along.

Both Percy Jackson and the Olympians and The Kane Chronicles started by focusing on a typically villainous figure from their own mythology before revealing them to be Good All Along and an Unwitting Pawn respectively. Similarly, the series will begin with him setting off a set of events that leads to the Big Bad beginning their plan.

The protagonists will be Like Brother and Sister
Percy Jackson and the Olympians had Percy and Annabeth, who eventually became a couple (although it took them long enough), while The Kane Chronicles had Carter and Sadie, who were brother and sister. The Norse protagonists will be friends but very clearly NOT interested in each other. They will do the "blood brothers" ritual at a key point in the first book.

Magnus will be related to Annabeth
We are not given much information on Anabeth's stepmother or her mortal siblings. Plus there's the fact that they have the same last name.
  • Well in the Blood of Olympus it's revealed she has a Cousin and an Uncle in Boston
  • Confirmation of this in the preview by literal appearance of Annabeth is a reason the fandom has exploaded

If Percy shows up in a crossover related to this series, he will complain about his pantheons lack of god killing snakes.
(Percy): The Egyptians have a snake that is supposed to eat Ra, you guys have a snake that is supposed to mutually kill Thor, why don't we have a god killing snake? (Magnus or Nico): You say that like it's a bad thing.

The series will end with Ragnarok being narrowly averted.
This seems to be a running plot in all of his series' where that mythology's Armageddon almost comes true.

All Myths Are True means that none of them are true.
As Riordan's series continue to develop, we see that more and more of the various cosmologies, be they Greek, Roman, Egyptian, or Norse, all exist in some fashion, complete with their component gods and otherworldly locales. This creates some noticeable issues when various cosmologies overlap, such as Norse mythology and Greek mythology both having their own thunder gods, their own separate afterlives, and so on. On top of that, we see that the gods' forms are mutable, that they change their appearance and demeanors to reflect the modern age, as opposed to what they were originally. This suggests that, contrary to popular belief, it is the humans who created the gods and it is human belief that both sustains and shapes their existence. The gods don't actually exist in and of themselves, but are born from the collective consciousness of humanity itself.