Niddhog and his brood as Night Dragons?
by Marc SaindonGaz7 (p.63) mentions the world-tree serpent Nithog the Dread Biter and his spawns, Gnaw-Fang, Grave-Root, and Root-Ripper.
Perhaps they can play a more active role in the Northern Reaches as dragons. Night Dragons, perhaps. With Niddhog at least on par with Glantri's Synn, and his children serving as his agents (one for Vestland, another for Ostland, and the third sabotaging the Soderfjord Jarldoms). These lesser Night Dragons often work through pawns, cults... and cross-purpose against each other.
Niddhog himself is busy trying to destroy Yggdrasil by poisoning its root (to serve the Sphere of Entropy), a feat he will never succeed at since Ragnarok foretells that it is the fire giant Surtr that will finally burn it down. Although, once that happens, Niddhog is free to roam the world and cause havoc after Ragnarok is completed:
"Finally, the völva prophesies that a beautiful reborn world will rise from the ashes of death and destruction where [Baldr] and [Höðr] will live again in a new world and where the earth sprouts abundance without sowing seed. The surviving Æsir reunite with [Hœnir] and meet together at the field of [Iðavöllr], discussing Jörmungandr, great events of the past, and the runic alphabet. A final stanza describes the sudden appearance of the [dragon] [Nidhogg], bearing corpses in his wings, after which the völva emerges from her trance."
That sounds like Final Boss material.
Access to Niddhog's lair, a dangerous undertaking, is possible by traveling the Great Salt Marsh in the Soderfjord Jarldoms and finding weak points in the veil between worlds, which would bring the characters into a Shadowfell sort of Realm. While Niddhog has power and fate on his side to prevent being destroyed by the PCs, he still can serve as a source of forbidden information. And depending on the power level you're willing to give him, he might actually serve as a Patron for the 5.5e Warlock class (either Archfey or Fiend for the mechanics) or even as a distant ancestor explaining a Draconic Sorcerer's origin.
Image from: https://www.deviantart.com/irishrng/art/Yggdrasil-55367111
