fuindordm
Adventurer
If I were playing in such a game, I'd like to see magic reined in -- maybe even forbidding evocation spells. But there was a lot of powerful magic in the stories too that I would like to see come into play eventually: polymorph, powerful curses (e.g. the nidhing staff), illusions, and of course legendary magic items. I would allow spirit barbarians, eldritch knight, arcane trickster, multiclassing, and the magic initiate feat, since the eddas abound with heroes who know a little magic. I would go through the spell lists of cleric and wizard and prune them down to remove spells that I consider too flashy, or change a couple to make them fit the stories more closely (e.g. Charm Person requires the target to accept the material component as a gift, but if it works the target doesn't realize they were charmed). The rest is flavor--a human cleric casting spells pulls out a rune-inscribed amulet, a paladin smiting smears a little blood over one of the runes on her weapon, etc.
Regarding the OPs question, I'd probably equate the dark elves and light elves with fey of all kinds. "Dark elves" could include goblins, kobolds, mythical dwarves (including duergar and derro), mythical gnomes (inc. svirfneblin), and all manner of malicious fey living underground. I would make svartalfheim equivalent to the Underdark so that it can be accessed without planar travel, and toss in whatever creatures I needed for the adventure. BUT, I would reskin the monsters as necessary to preserve the Norse feel. In short, whatever monsters the characters encounter I would call "dwarves", and the different game statistics represent different tribes of dwarves. The "Light Elves" would be a catch-all for generally good (or at least not evil) fey, and Alfheim would essentially be the Feywild.
So you have three easily accessible worlds: Midgard, Svartalfheim (by finding the right tunnel), and Alfheim (by finding the right hillside cave, stone circle, fairy ring or whatnot) that adventurers can go to without higher magic. And to the adventurers, all the monsters of Svartalfheim and Alfheim are just "dwarves" and "elves" with bewildering variety, and they're probably all shapeshifters in any case so why bother trying to classify them?
Same for trolls--the standard MM troll is actually a poor fit in my opinion (except for the long noses) and the term can encompass anything from orcs to ettins, as well as hags. They probably look more similar to each other, but orcs I would just describe as a band of small trolls.
Giants work well as-is, but I would give their leaders more spellcasting ability (mostly illusions).
The PC races are all basically humans with a touch of the right ancestry, and they all look more human than not. For example, a half-orc (trollborn) is probably big and muscular, and maybe really hairy and/or ugly, but doesn't have an obviously nonhuman trait. Dwarves and gnomes are probably on the short side and dark-skinned, but that's it.
Dwarf or gnome: your family has some dark-elf heritage.
Elf or halfling: your family has some light-elf heritage.
Half-orc: your family has some troll heritage
Half-elf: changeling children or huldra
Shifter (see Eberron handout): skin-changer
Good luck with your game! I did something similar ages ago using the Hero system, and the campaign lasted 4 years. The Scandinavian world had lots of little kings and jarls who would try to attract heroes to their service, which is a good setup for getting together an adventuring party. And if you don't know it, check out Poul Anderson's "Hrolf Kraki's Saga" for some inspiration.
Regarding the OPs question, I'd probably equate the dark elves and light elves with fey of all kinds. "Dark elves" could include goblins, kobolds, mythical dwarves (including duergar and derro), mythical gnomes (inc. svirfneblin), and all manner of malicious fey living underground. I would make svartalfheim equivalent to the Underdark so that it can be accessed without planar travel, and toss in whatever creatures I needed for the adventure. BUT, I would reskin the monsters as necessary to preserve the Norse feel. In short, whatever monsters the characters encounter I would call "dwarves", and the different game statistics represent different tribes of dwarves. The "Light Elves" would be a catch-all for generally good (or at least not evil) fey, and Alfheim would essentially be the Feywild.
So you have three easily accessible worlds: Midgard, Svartalfheim (by finding the right tunnel), and Alfheim (by finding the right hillside cave, stone circle, fairy ring or whatnot) that adventurers can go to without higher magic. And to the adventurers, all the monsters of Svartalfheim and Alfheim are just "dwarves" and "elves" with bewildering variety, and they're probably all shapeshifters in any case so why bother trying to classify them?
Same for trolls--the standard MM troll is actually a poor fit in my opinion (except for the long noses) and the term can encompass anything from orcs to ettins, as well as hags. They probably look more similar to each other, but orcs I would just describe as a band of small trolls.
Giants work well as-is, but I would give their leaders more spellcasting ability (mostly illusions).
The PC races are all basically humans with a touch of the right ancestry, and they all look more human than not. For example, a half-orc (trollborn) is probably big and muscular, and maybe really hairy and/or ugly, but doesn't have an obviously nonhuman trait. Dwarves and gnomes are probably on the short side and dark-skinned, but that's it.
Dwarf or gnome: your family has some dark-elf heritage.
Elf or halfling: your family has some light-elf heritage.
Half-orc: your family has some troll heritage
Half-elf: changeling children or huldra
Shifter (see Eberron handout): skin-changer
Good luck with your game! I did something similar ages ago using the Hero system, and the campaign lasted 4 years. The Scandinavian world had lots of little kings and jarls who would try to attract heroes to their service, which is a good setup for getting together an adventuring party. And if you don't know it, check out Poul Anderson's "Hrolf Kraki's Saga" for some inspiration.