In many ways, the Assassin's Creed franchise has always been about immortality, or extending life beyond normal means. The original Ezio games saw the memories and words of Altair live on in Ezio, and the actions and lives of the two mentors live on in Desmond. The ancient Isu who helped him (Jupiter, Minerva, and Juno) projected themselves through time; their entire job during the human-Isu war was preventing/surviving the first Great Catastrophe. Indeed, their machinations resulted in several methods that focused on surviving this seemingly naturally occurring, evolutionary event.
Of course, the second Great Catastrophe is prevented thanks to this research, extending the life of the Earth if releasing Juno into it (just to be killed in an Assassin's Creed comic book). Now, the latest games have been trying to find a timeline where yet another, then yet another, and then yet another Catastrophe does not try to occur. In other words, while the lore of the game seems to be pushing it toward yet another disaster, the stories of Assassin's Creed Odyssey and Assassin's Creed Valhalla capture "immortality" the most poignantly. SPOILERS FOR BOTH GAMES AHEAD.
Assassin's Creed Odyssey
Despite all the aforementioned lore bits and the idea of how one soul can live on in another (in the case of Ezio in Desmond), there's been no "true" representation of immortality until Assassin's Creed Odyssey. Now, generally-speaking, immortality does mean to never cease existing and to never die, but the general consensus is all that living a really long time counts for immortality too. The canon Kassandra knows something about this. Wielding the Staff of Hermes, Kassandra is revealed to have lived 2400+ years. She lived until she could pass the staff on to Layla, at which time she seemed to pass peacefully.
This is really a direct representation of that; players then continue to see Layla serve as this Keeper who lives on through Kassandra's eyes in Atlantis, training her own abilities. But it is this establishment of the Staff of Hermes that comes to the fore in Assassin's Creed Valhalla, where it's not just one soul having extended their beyond normal means, but several.
Assassin's Creed Valhalla
This idea of immortality pervades AC Valhalla; through Eivor's eyes and mythological beliefs, players see how Odin stole the 7th method of surviving the Great Catastrophe from Jupiter, king of Jotunheim in this comparison of sorts. Indeed, the original Isu trio are the ones who are portrayed as the Jotuns, as Odin's faction of Isu differed from the Greek. Nonetheless, Odin did steal it, and the Norse Isu did use it.
As a result, fans see Odin attempting immortality by being reincarnated as Eivor, Loki as Basim, Freyja as Svala, Tyr as Sigurd, and more. Fans will also know that Eivor has clearly died, seeing her body in the modern day, but that begs one big question: has Odin been reincarnated again? Have any of these figures? Or is it a one and done deal? Because if the escalation from Odyssey to Valhalla, as well as other events that unfold make clear, is any indication, this pursuit of immortality only seems to be beginning.
Layla loses the Staff while inside the Yggdrasil device, losing her physical body but joining the Reader in attempting yet another disaster. The Reader is a figure who has, too, extended his life beyond normal means. The Reader is clearly Desmond, though it seems he has forewent everything in pursuit of a singular goal. Even then, as Loki comes back to life and his plan plays out, it seems he is intent on somehow resurrecting Aletheia who has extended her life if not physically. Loki, of course, is alive again, and there's a ton of questions of what's going on.
In short, while it remains to be seen when the next game releases, fans should keep an eye on these themes. It cannot be confirmed, but it sees likely that this newly forming series of games has a huge emphasis on immortality, a potential unifying theme in the ongoing saga.
Assassin's Creed Valhalla is available now on PC, PS4, PS5, Stadia, Xbox One, and Xbox Series X.