The sword calls itself Jack. It may sound ridiculous until you consider jackknife. Not that bad now is it? It's also a shout out to "Jack the Giant Killer".
Hearth wears all black because black absorbs sunlight. Blitz mentions that the scarf was his idea and that Hearth used to wear only black, so it was a conscious decision.
Not sure if it counts, but the author's decision to make Samirah of Middle Eastern origins may add another potential dimension of why people irrationally mistrust her (along with her parentage) as expanded from the memory scene with the bullies in grade school.
When Odin reveals himself as X at the end of the book, at first it seems to come out of nowhere. Then you remember that it was mentioned early on that X was brought to Valhalla by Samirah, a decision that got her in trouble. And who was she secretly working for?
Crosses over with Genius Bonus. When Jack meets the goat Otis, he says that they met at a party where Otis played Tug of War with Loki. Otis says that that party was embarrassing. Why? That party was the one where Skadi challenged the gods to make her laugh. Loki made her laugh by tying his testicles to a goat's beard and having them thrash about. If Otis was that goat, no wonder he was embarrassed...
It's implied that at least part of Heimdall's Phoneaholic Teenager act is a response to being able to see and hear EVERYTHING in the Nine Worlds. Note that in chapter 32, after he puts down the phone, he gets more serious. If you had senses that acute, and had constantly been standing sentinel for centuries, you'd probably a) go a bit bonkers and b) want to experience everything through a phone screen rather than your own senses.
Compared to the Kane siblings, Magnus interacts with the Greek world a lot more. Other than Annabeth being related to him, there is a reason for this: the Byzantine Emperors of Eastern Rome would hire Norsemen as bodyguards. They were called the Varangian Guard. The world always had some ties, particularly in the East (where the Greek Camp is always located).
Additionally on the Kane Bit, the crossover elements became less subtle at the end of the series, while the connections to Greece were always stronger in Magnus. The Egyptian history dates back ages before the time of the Athenian/Spartan Greeks, ending with the New Kingdom and most relevant parts of the history the Kanes delve into, bar the dynasty of Cleopatra, long before most of the Greek history learned in schools (in fact, with Apollo giving his age in the Trials books, the Old Kingdom actually began before Apollo and Artemis were ever born). The Greek connections only were strong at the end of the series, with the post-Alexander kingdom and some New Kingdom era trading connections prior to the Great Bronze Age Collapse. Germanic tribes, to whom the Norse were tied, had a much longer and less interrupted interaction period with the Romans and Greeks.
At one point, Magnus compares Andvari's ring to the more famous One. While this may seem like nothing more than Magnus being his usual Pop-Cultured Badass self, the fact is that J.R.R. Tolkien had a great love of the Norse myths. It's entirely possible that the One Ring is based off of Andvari's ring.note Word of God, as noted here, says that The Ring of the Nibelung didn't influence his magnum opus, but it's hard to imagine that the original myth wasn't a conscious or unconscious influence.
In the second book, Alex mentions that s/he is gender-fluid so often that it came off as annoying to some people. But by the third book, s/he doesn't mention it as much and Magnus doesn't have as much pronoun confusion. Author's Saving Throw? One part, but one other part is that not only does the narrator (Magnus) used to it, but so are the readers.
The Einherjar are only selected from people who die saving other people (and holding a weapon), rather than anyone who dies in battle. This is because they will all die when they fight in Ragnarok, and the only ones who are certain to be able to die for someone else are the ones who have already done it!
Sam plans to retire from the Valkyries when she's 18. Given that the Valkyries don't age while they're Valkyries, she's probably trying to resume aging before the age gap is too noticeable.
So Loki, the Big Bad, is beaten not by an epic battle, but by a "The Reason You Suck" Speech. Seems somewhat like a copout considering that the vikings were warriors - so why not defeat the Big Bad of Norse Mythology via a duel to the death? After all, compared to Kronos, Apophis, and Gaea, who else would get an epic Final Boss fight than the Norse? Well, it makes perfect sense - Loki is not a war deity, but a trickster deity. What better way to trick warriors than to require charisma, a frequent Dump Stat of fighters.
Even when one looks at the group and notices how much like a standard RPG party they are, it's the Healer who beats Loki.
A flyting is a verbal duel of insults, where each participant displays their prestige, power, and confidence. Nothing in the rules says it's limited to 'direct' insults though, or 'individual' prestige/power/confidence. By adding his group's feats together, Magnus boasts power that rivals or even surpasses the trickster Norse God. With his words - amplified by the mead - he speaks of a group who covers for each other, help one another, and otherwise keep each other from dying while Loki himself always stands alone. Friendship, trust, love, camaraderie, etc... all concepts and words that are an insult to everything Loki believes in.
Of course Riptide is female, what did the two original owners of Riptide have in common after all?
Riptide also started life as a hairpin, a woman's accessory.
It's not really dwelt on, but there's another reason the Hotel Valhalla staff don't use AD or BC, for the same reason that many non-Christians don't use them. It's just irrelevant when you don't worship Jesus.
By the same token, Sam tells Magnus that "the full moon on Wednesday" means Tuesday night, which she understood immediately. When you follow a lunar calendar, as Jews and Muslims do, the "night of" always means the night before.
Loki and Hades make an actually interesting compare and contrast. Both are gods who are commonly treated like Satan in other media and have bad reputations. Both are tied to the realms of the dead and both are distant but crucial to their mythos. Gods like Thor and Apollo mention them in happier times wistfully, when they weren't as antagonistic. But Hades seems to have become as distant as we see him since the 1940's when Maria was killed. He became more distant and bitter after that time, and after the wound was shown to heal in the aftermath of Last Olympian he becomes more open and positive to Nico and the other gods. Loki however had his 'Maria' moment centuries ago with the sons whose guts were used to bind him. He had centuries to wallow in his bitterness with the positive traits Thor misses and Sam and Alex mention becoming drowned in his negative qualities. Loki is what Hades would have become if the wound of Maria's murder by Zeus had had centuries to fester.
To expand the comparison the Oracle that Hades cursed could easily have become his Baldur moment if it had truly gotten out of hand. The moment he as a god caused such legitimate and near-permanent damage that the other gods never forgave him.
However, the vital difference is that Hades cursing the Oracle is one of the very few negative things he's ever done (that we know of). In the original myths, he's usually presented as being more responsible and bound by duty than most of the other gods. Notice, for example, how few demigod children he's ever had; he's not the kind to stray from Persephone (most of the time), nor does he ever shirk his duty as judge of the Underworld. Loki, however, had a compulsive need to show off, trick, and annoy the other gods, which he would ameliorate by helping them out and making things better (usually after one of his clever plans went south). The Greek gods are much more willing to forgive Hades' one-time act of cruelty; with Loki, sooner or later he inevitably would've done something too bad to forgive.
Fridge Horror
In The Ship of the Dead, the final chapter showcases a brief phone conversation between Magnus and Annabeth, wherein Annabeth says that she's glad to get some good news with indications that something terrible has happened on her (the Greek) end of things. It doesn't seem to be alluding to a specific event that has occurred in any of the currently-published books, which seems to suggest that the next book of the Trials of Apollo series will feature a particularly tragic development.
And as of The Burning Maze, we do in fact see what made Annabeth so upset. The death of Jason Grace.
Magnus neglecting and ignoring Jack in The Ship of the Dead is mostly Played for Laughs, but given that we know that Sumarbrandar will one day be wielded by Surtur, it's quite possible that for Jack, it's more of a Start of Darkness...
. The world always had some ties, particularly in the East (where the Greek Camp is always located).