I was doing a little research into the Anglo-Saxons since they were recommended as a civ for this earlier on, by Highmax as I recall, and I was a little surprised at how few differences there were between their armour and weapons and those of the Norse. I shouldn't have been surprised of course as I've known that the Saxons were of nordic stock since I was a child, but I would have though the Angles would have had more of an influence on the weaponry.
In any case, their equipment consisted of multiple spear variants, more than I can be bothered with to be honest, hand axes, throwing axes, bows, spatha swords, various helmets and a small mail shirt, though the shirt was supposedly quite rare and expensive. Throwing axes can be implemented, though they'd be really clumsy things and serve no real use as they'd function the same way as bows at range. Everything else is doable though.
EDIT: Some more ideas: Nidhoggr dragons, cave dragons altered to be more like the norse dragon currently in the mod. Based on nidhoggr obviously, able to fly, breathe fire, antlers rather than horns as a depiction of nidhoggr I liked used antlers, not megabeasts since I don't know how that works with caverns. Would like it to have blood that does something other than grant nigh-invulnerability, not sure what though.
Heljotun: Either a civ or a megabeast, based on the undead giant that guarded Helheim and took the nails of the dead to build the ship of nails. Would serve as an undead corpse animating monster of prodigous size.
Fimbulwinter wolves: If we assume due to the giants marching against the humans in such numbers this mod could be placed somewhere around Ragnarok, then the Winter of Wolves seem a decent thing to include. Large wolves that attack in winter, maybe with some kind of worldwide weather effect that happens in winters.