Thanks for your thoughts. Happy to hear anything further.
Because of my discomfort with cultural appropriation, as well as my lack of solid Norse history, etc I decided to go with the setting being Vanir-derived. As I understand it, the Vanir are a giant question mark in a lot of respects, so I feel like I've got room to play.
Different cultures have different sensitivities. For Norse, appropriation is welcome. Also playfulness, fun and humor are normal. On the other hand, accurate representation is important. For example, dont portray vikings as-if Conan the Barbarian, because this is inaccurate and many people do find it disrespectful. If unsure about something, dont call it by the Norse name. For example, it is fine to have a loosely inspired Valkyrja-esque concept, but then call it a "Fatalist" rather than a Valkyrja.
The Vanir are Norse, but correspond Saami and Finnish concepts. They especially relate to burial customs: Freyja is the main carer for those in graves, and a collector of half of those who die in combat. Freyr relates to the horse meat that the dead are buried with. Njörðr relates to the ship burials and ship-shaped burial stone markers: the sea brings pleasant warm climate to the near arctic.
To represent the Norse concepts, the æsir and vanir, and alfar and dvergar, and jötnar, are natural features of the Material Plane. These natural influences project across the Ethereal Border. They arent human but sometimes take the form of a human as an avatar. The Ethereals observe the overlapped Material Plane, and the powerful ones can manifest physically into the Material Plane.
Altho these nature beings are Ethereal, the corpses of the grave have both a negative aspect that Hel presides over, and a positive aspect that Freyja (with other æsir women) presides over. In other words, the underworld divides up regionally, where some of it is Shadowfell and some of it is Feywild.
D&D level advancement works normally, and any nature being (æsir, alfar, etcetera, including humans) can be low level while certain individuals are high level, even epic.
Basically, the setting is Norse-adjacent. Freya has her own sort of Valhalla (Fólkvangr) and had to marry Odin to calm down the war between the Norse and the Vanir. However, Freya was also aware, in something of a meta fashion ('cause of magic yo) of the role the Norns played in locking her into an endless cycle of repetition. So she decided to try and break it. Fólkvangr became a home/refuge, not just for Norse and Vanir dead, but also for others from across the planes. Some of Loki's get also found a home in Fólkvangr. The ... Valkyrja (is that the proper/better term for what we usually write as Valkyrie?) were more than mortals but less than gods. And, over time and outside their "normal" selection and ferry duties, they also had families. The off-spring of these unions were also extra-heroic, given who their parent were.
Avoid the English term "gods". Under Roman Empire influence, Germans worshiped them, but the remote Norse never did. The Norse term "goð" means an "invoked one". It is ok to ask an æsir or jötnar or vanir or dvergar or alfar or human ancestor for help, but it is the same as asking a neighbor for help.
Seithr (my chosen anglicisation of Seiðr) was widely practiced in a variety of forms and held an important part in the lives of... Fólkvangrians?... both those that were chosen from the slain, as well as the offspring of Valkyrja.
Seiðr is specifically mind magic: D&D telepathy, enchantment, domination, suggestion, phantasm, and by extension objectively real illusions. Subjective visualization and objective reality are a blurry boundary, and a "strong" mind can will reality into existence.
Fate magic (divination, luck, bless, etcetera) is Spa.
Warrior magic, called Ljóð "songs", is much like honorable Paladin protective magic. If a Bard can do it, it is probably Norse too.
Eventually, Ragnarök arrived, as it always did and a chunk of Fólkvangr emptied out. However, those that had been born there, those that had sought and been granted refuge... they were immune to the call. So Freya's plan sorted worked. Before Ragnarök, of course came Fimbulwinter. When Fimbulwinter descended, everyone worried because... ya know... death by freezing. However, it turns out that the svartálfar were more clever than folks thought and previously unknown/sealed passages opened up and provided underground refuge for the duration of Fimbulwinter and Ragnarök.
These passages and underground refuges twined through the roots and branches of Yggdrasil and weren't always the safest of places. Explorations seemed to indicate that they went to other realms, and it was speculated that the passages utilised Yggdrasil in some fashion.
The svartálfar themselves were never seen and it's unknown if they were simply allies of the Norse/Vanir, or also subject to the Norns. Over time, as practice of Seithr evolved, new areas were unlocked and new learnings were gained.
Eventually, in some unknown fashion, the refuges which had been sealed against Fimbulwinter and anything that might have survived outside, opened to a land that had once been known but was now reborn into something different.
So... yeah. Kinda post-apoc but not. More hopeful than post-apoc tends to be. Instead of "everything used to be great and then they broke the world and now it all sucks", it's "things were good, Fimbulwinter sucked, and now we've got the tools to really make the world different".
The world is a giant unknown and characters will be exploring to fill in the gaps, make new alliances, etc. Old ruins to explore (thanks Fimbulwinter), monsters and stuff to fight (thanks Yggdrasil, svartálfar refuges, and crazy requests for sanctuary that were granted), and all the other traditional adventuring stuff. Most of the setting is unstated, free for whomever to populate however desired. I've got an idea for a map, which I may or may not include.
All of this is in the back of my mind as I'm working through the rules base. So a ...Valkyrja?... class would be nice in terms of tuning the rules to match more with the setting, but... it's not a make-or-break thing at this point.
A Norse setting that takes place in the present or the future is a good idea, and helps avoid the problem of misrepresenting the past. For example, taking all the tropes from Norse sagas and eddas but remixing them in new ways that happen in the future, feels fine. There is no intention to represent the Norse during the Viking Period. The character might be a grandson of Þórr rather than him himself, so there is artistic freedom for how to characterize the grandson.
