Giant Units
It would be really interesting to see giants grow, rather than being built, I think the possibility for it exists within the game mechanic. They would make a builders civ - small cities, growing units, building buildings and research. Birth can be emulated with conditional spawning - I'll try to look through XML/python and see how it is done for other civs. Growth can be emulated with XP growth (like workers in FF), so that Younglings are really slow work-rate workers, but once upgraded they change unit class and evolve according to different rules later.
Instead of saying spawn - you could say "Rejoice! The news of a youngling born in <staedding> has reached us."
Egrass the Founder - the giant protector and worker, while the Homestaed is being built and offspring are growing, he works tirelessly to improve the lot of his kin.
Hearthmother (2/4/4 - visibility range) - it was the women of Jotnar that took to Egrass's ideas best, who raised the new generation of Giants with need to build, explore and learn. While Giant men built and warred their women ran their staeddings, and soon no decision of importance was taken without Hearthmother's consent. (The unit could have improved vision - all starting with a first settler bonus - to show their ability to scry the futures)
Youngling (1/3/1 - sentry) - the children of the giants are rare, but only the most remote staeddings in harshest lands hear not the Youngling's voices. Not built but spawned by the "Jotnar Staedding" building if the staedding has a grown Giant in it for more then 20 turns. They are spawned with a single random promotion to show what their inclinations are (Hero, Arcane, Mobility, Forest, Hills, Combat I, Marksmen, etc) The more specialized ones should be pretty rare. Younglings should really stay with their staedding until they have grown. That can be achieved with a removable Heavy promotion. They can age with XP - as soon as they are 10 or 17 xp level they can be upgraded based on the techs and buildings available. The options would be Skald, Hill Dweller, Worker, and Hearthmother.
Hill Dweller (7/7/1 - sentry) (axeman replacement) - the civilized counterpart of their wild kin Hill Giants, these are the people of Jotnar, protectors of staeddings and fathers of younglings. They are prone however to the call of the wild. When the freedom of the hills calls them with the ancient fury of gods battlefields they gain a a crazed and hidden nationality promotions. (Can be achieved through a spell, or maybe just a random possibility like werewolves and lunatics).
Jotnar Worker (2/4/2) - while most giants choose to leave their home staedding to build their own, some choose to remain with their family and become workers, they build routes between staeddings and create improvements - highly praised and respected they follow in Egrass's footsteps.
Skald (6/6/3 - sentry, channeling two, medic, channelling I, (nature I, earth I, fire one, mind I - something to reflect the basics) - both men and women of Giant kin are skilled in the arts, Skalds are shamans, poets and healers - they travel from staedding to staedding learning the lore in their travels, providing vital news to their kin and are welcomed by all for their many stories. They learn slower then their human counterparts, but can gain experience in combat.
The graphic probably should be gender unspecific - like a enlarged Mage - covered in cloak - maybe other mods have something like that - I'll look.
Wielder of Arms (11/11/1 - sentry) - These formidable opponents are giants that were forged in the new times of war, they are provided with armor and mace and protect the border towns and wage war for Jotnar. Any Hill Dweller can answer the call and become the Wielder of Arms. (Perhaps they should be unbuildable - but upgraded from level 3 Hill Dweller, or upgraded from Hill Dweller with a building present (Armory or something like that).
Vala (5/10/1 - sentry, channeling III, enchantment 1, law 2 - upgraded from level 6 Skald) - Vala are the wisest women of Jotnar, they scry the fates of their people and decide when they go to war or settle in peace, they practice magic lore and while their weapons are wands and not spears - they are formidable defenders. They can also call the Jotnar man to battle - inspiring valor and loyalty and enchanting their weapons.
Woodfather (10/10/2) - The Jotnar druid.
Titan - giants do not grow much in human eyes - big is big, but when centuries pass, the wisest and strongest become immortal Titans, those who hold the world on their shoulders.
The Kin Units
The Kin Units would be all the other creatures and giant related units - rather then being built, I think they can be gained like Sheaim or Kahdi mechanic. You have to have "Kin Fire" building in the staedding and then replace buildings that usually give those units to other civs with flavored ones. For example Wild Trolls just need Kin Fire to spawn, Hunting Lodge and Kin Fire let you get Hunter Troll and upgrade your Wild Trolls to Hunter Trolls, Kin Fire & Lighthouse would give you Sea Serpents & Turtles, Kin Fire and Harbor - Triton, Cyclops - Kin Fire and Grove - making rangers appear later in game - or maybe there's a way to associate spawn with technology.
Instead of saying spawned you could say: "A <unit> has been attracted to the Kin Fire in <staedding>."
Wild Troll (4/3/2 - crazed, hidden nationality)
Hunter Troll
Troll Elder (cannot be spawned needs to be upgraded from lvl 6 Hunter Troll)
Cyclops
Sea Serpent
Sea Turtle
Triton
Not quite sure

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Jotnar Berserker, T4 units in general. Seems like their T3 units are pretty powerful, and if you give them ability to get random promo at birth including a rare hero - they might be up to task to fight with human opponents without excessive T4 ranks, or you could get them through the Kin mechanic.
Would be cool to see "
Drakes" as air unit for giants. Their castles are big, Jotnar are big - Drakes are like hawks to them. And my desire to have flying lizards of some sort would be satisfied

Maybe along the lines of Rock Ravens of Chislev, but with ability to walk 1, like a 5/5/1 flying, and fireball tossing miniature dragon.
Race - maybe add race "
Giant" - and give it to Giant Jotnar units. It might be possible to pass on the race promotion with upgrade - make Hill Giants upgradable to Hill Dwellers. That would allow them to be fortified, since beasts cannot be fortified. Also might make it easier to work with unique spells and events.
Unique Spells
Staedding Call - a domination type spell - 25% chance of converting a Hill Giant. Should be available to Egrass, Vala, Woodfather and maybe Hearthmother. The learned and strong Jotnar can seek out on their own risk their wild counterparts and get them to join the civilization. It would make a lot of fun and a game of chance if Hearthmother goes on her own to found a steading and can get herself a man by calling to him.
Game Mechanics
Mainly I thought that conversion between Hill Dwellers and Hill Giants can be more pronounced. Captured Hill Giants can stay as they are, or be upgraded to Hill Dwellers. When they are Hill Giants they are savage beasts and can attack anyone, and possibly be crazed with a chance to turn wild. When they are Hill Dwellers they lose hidden nationality and gain Giant race, but have a chance to become Hill Giants with all the effects of it - HN & Crazed. Shouldn't be too high of a chance of it happening - maybe 5% chance each turn for Hill Dweller to become Hill Giant.
Armageddon Hero - I really like the idea of waking up Ymir when the counter is high up - say if it's over 70, Jotnar can build a really costly huge giant to protect them.