Fans are eagerly awaiting the next Assassin's Creed installment, currently expected to arrive sometime later this year, but Ubisoft has yet to formally announce the game (much less when it'll be released). As such, all fans have to go off of right now is a series of leaks which supplied the game's rumored title: Assassin's Creed Ragnarok.
Further leaks have left room for speculation as to what'll be included in the plot of Ragnarok; right now, it's looking like the player character is someone named Jora--who can be male or female, players' choice--and the game takes place during the age of Vikings, circa the early 1000s. Given that, Assassin's Creed Ragnarok has the potential to play with plenty of truly fascinating Norse lore, both real and mythical.
Valhalla
When editions of the upcoming Assassin's Creed game were accidentally leaked on Amazon and Gamestop, it suggested that there would be versions of the game called "Valhalla" and "Mjolnir." Whether or not that's true, both could easily be involved somehow in Ragnarok.
Valhalla can be likened to a Norse version of heaven or the afterlife: it was said to be a great hall located in Asgard (the god Odin's realm), where great warrior Vikings who die in battle go after their deaths, shepherded there by valkyries. Valhalla's ceiling was made from thatched golden shields, and the golden tree Glasir stands before the enormous hall. Considering that the final Assassin's Creed: Odyssey DLC, Fate of Atlantis, transported players to another realm, that definitely leaves Ubisoft's options open with Ragnarok.
Plus, the warriors and kings who ended up in Valhalla supposedly began making preparations to aid Odin during Ragnarok. If Ragnarok is really the game's title (or if it involves Ragnarok regardless), then all of the fabled figures associated with it are fair game.
Mjolnir
It's interesting that "Mjolnir" was the other leaked version of AC Ragnarok; Ragnarok is an event, Valhalla is a place, and Mjolnir is, of course, a famous weapon in Norse mythology. Marvel fans will know it right away since Thor spends a good chunk of his screentime in the MCU wielding Mjolnir, but for those who need to brush up on their lore: Mjolnir is the hammer of Thor, the god of thunder, and Norse mythology regards it as the most powerful weapon in existence--capable of grinding entire mountains into dust. Funny enough, the Prose Edda also tells a story about how Mjolnir's handle is oddly short compared to the rest of the weapon, due to an error when it was crafted.
Mjolnir's involvement in AC Ragnarok could be a couple of different things: on the one hand, perhaps the player character Jora will get to wield a hammer with this name (although it probably won't be the mountain-toppling thing that Norse lore suggests). On the other hand, maybe Mjolnir will be as powerful as the myths say, and it'll be a powerful artifact that needs to be kept from enemy hands. Perhaps it could be a Piece of Eden.
The Nine Realms
In Norse mythology, Earth was part of a configuration called the Nine Realms, over which Odin and other gods presided and protected. Earth was referred to as Midgard, and beyond that, there was Asgard, the realm of gods; Alfheim, the realm of bright elves; Jotunheim, the realm of the giants; Muspelheim, the realm of fire and the giant Surtr; Nidavellir, the realm of dwarves; Niflheim, the realm of ice and mist; Svartalfheim, the realm of dark elves; and Vanaheim, the realm of the Vanir.
As mentioned before, Norse mythology definitely gives Ubisoft the option of doing another Fate of Atlantis-esque adventure for AC Ragnarok. Even if that doesn't happen, the Vikings' Nine Realms theology could present itself in other ways within the game.
Odin the Allfather
Norse mythology can't be talked about without mentioning Odin the Allfather, leader of the gods, and husband to the goddess Frigg. He's normally associated with a lot of things within the pantheon, including wisdom, royalty, battle, war, and victory. Since the Vikings of AC Ragnarok are likely very much at war, at the very least, Odin's name could be invoked during battles. Or, perhaps, he'll show up in some other form somewhere during the course of Ragnarok.
Since Mjolnir has also been brought up, perhaps Odin's son, Thor, could also make an appearance. The two are tightly linked, so it's hard to imagine a Mjolnir that isn't being wielded by the god of thunder in battle.
Ragnarok
Finally, there's Ragnarok itself: considering the upcoming Assassin's Creed game is thought to have "Ragnarok" right in its title, it'd be awfully surprising if Ragnarok (or Ubisoft's take on it, at least) didn't present itself in one way or another in the game. Ragnarok in Norse Mythology is the final days of reckoning for mankind--or, more simply, the destruction of the world. It's also referred to as Twilight of the Gods, since according to the many Norse tales, Ragnarok will be caused by a great fight between the Aesir (which refers to a pantheon of "good" gods, including Odin, Frigg, Thor, and Baldr) and the forces of Hel, led by the trickster god Loki.
Ragnarok's presence in Assassin's Creed may not actually mean preventing a literal battle between gods, but the concept of Ragnarok approaching as the world's ultimate destruction sounds like something that Ubisoft could make great use of in the game's plot. Perhaps the mysterious Pieces of Eden will have something to do with an Assassin's Creed-style Ragnarok, or perhaps an opposing faction could try and usher it in. Either way, if the game really is called Assassin's Creed Ragnarok, then there's a very good chance that players will find themselves facing down the Norse apocalypse.
Overall, there are a lot of places that Ubisoft could go with a Vikings-era setting, and Norse mythology as a whole offers an extremely wide range of tales to pick from. Vikings weren't just the battle-hungry barbarians that history paints them as; their culture was a rich one and would be fascinating to fully explore in Ragnarok.
Assassin's Creed Ragnarok is rumored to be in development now.