Rise of the Empire of Man
The Empire takes Kislev
In official GW lore, there is no mention of Karl Franz having a wife or kids that I'm aware of. He’s a handsome guy who needs an heir. Queen Katrin of Kislev is a beautiful woman who need an heir. Kislev repeatedly requires the armies of the Empire to rescue it from Chaos incursions. The Empire relies on Kislev being a barrier against Chaos. Karl Franz is a legendary warrior. Queen Katrin is a formidable sorceress. They really would make a great power couple. It just so happens that the Empire has ten provinces and twelve runefangs.
Maybe it was before or after Karl Franz, but at some point the Emperor of the Empire and Queen of Kislev marry. As a wedding present the queen of Kislev receives the Drakwald Runefang and their first born becomes the First Elector Count of Kislev.
In general the Kislevites are very happy to be part of the Empire.
The Empire takes Tilea
Tilea is a rich and diverse regions that is split between several feuding City-states. The Empire captures the city-states one by one. The Tileans do not think to unify until it’s too late. The wars are bloody but short as the Tilean cities usually surrender after fairly short sieges. To smooth over the transition, the emperor gives the most powerful TIlean noble family the Solland runefang. To placate that noble family’s Tilean rivals, two more noble families are brought into the fold. They don’t any official power really, but they do get to sit in on as electors much like the representative from the Moot.
In general the Tileans are moderately content to be a part of the Empire.
The Empire takes Brettonia
Brettonia is a unified nation with terrain that serves as a barrier to invaders along the Empire border. The Empire conquers Brettonia in a very bloody war that lasts at least ten years and a lot of the Brettonian nobleman are purged after the takeover.
In general, the Brettonians view the Empire as brutal oppressors.
The Empire takes the Borderlands
The Empire sends a modest army into the Borderlands. The Borderland princes are not morons. The Borderland princes agree to fly the banner of the Emperor above their own. They agree to pay the Empire of Man a modest tax. In exchange the Emperor gives the Borderland princes relative autonomy.
In general, the men of the Borderlands are mildly annoyed that they have to pay the Empire taxes when they provide next to nothing.
The Empire loses the Borderlands but gain so much more
The Warboss of the latest grand Waaaagh!!! Realizing that provoking the Empire of Man directly is suicide and basically butchers the Borderlands which the Empire doesn’t bother to defend. The Black Orc Warboss imposing a semblance of order on the region or what passes for order among greenskins.
Now the Dwarfs face a unified front from an orcish horde that is essentially unable to attack anywhere else and thus will not be distracted. In desperation, the Dwarfs basically agree to become a puppet state of the Empire of Man in return for protection and the combined armies drive the orcs back into the Borderlands.
The Empire takes Estalia
Before the first shot is fired, Estalia waves the white flag. Estalia is allowed to exist as an independent nation in exchange for a yearly tribute. Ulthuan follows suit but their tribute is much smaller and it is understood that the Empire of Man cannot get enough ships to make a serious go of assaulting Ulthuan, the High Elves just figure the token tribute is insurance against a two front war against Naggaroth and the Empire of Man at the same time.
The Empire cannot expand more
There is very little arable land in the Southlands to claim, and it’s far away. The Norse country is too close to the Chaos Wastes in order to hold for long, but a few foolhardy emperors attempt failed crusades to take these territories anyway. The Empire sets up forward bases and outposts in Norsca and the Southlands but they cannot really be said to control these places.
As a side note, the Vampire Counts of Sylvania are still around collectively decide to keep a low profile to avoid the Empire’s wrath.
Fall of the Empire of Man
The Empire of Man beats back the Chaos hordes, possibly forever, but their land is literally warped and at best one in ten people survive the Chaos War and its aftermath. That’s not enough men to support a massive imperial beuarcracy. To make matters more difficult the emperor and a lot of the electors died making succession messy.
Brettonia rebels almost immediately and even seize the Port of Marienberg. This prompts Estalia to stop sending the Empire tribute. Most of the devastated provinces of the Empire secede and become independent kingdoms.
The vampires of Sylvania try to make their move to use all the massive amounts of dead from the Demon Wars to raise an army and reform the Empire of Man as the Empire of the Dead, but there is a major problem. Magic is beginning to weaken. There are plenty of corpses to convert into soldiers but not enough magical energy to awaken more than a fraction of the dead. The initial vampire aggressions are repulsed and the remaining vampires start fighting amongst themselves as magic remains scarce. Those vampires that are not destroyed scatter though out all the emerging human nations and resume quiet lives where they pretend to be normal humans and hide the bodies of their victims very carefully.
What once was a solid block of land is now jagged and separated as the ocean cuts across the lands of the Old world created new coastline. Middenland is now a coastal province. Nordland is now half the size and is connected to the rest of the old Empire by a fairly narrow Isthmus. Ostland also lost land and gained coastline.
The land of Norsca, once in the thrall of Chaos are freed from it, at least temporarily. They ally with the new independent kingdoms of Nordland and eventually merge. They take the name Greater Norsca because I believe Norsca is a cooler name than Nordland. Together they become the dominant sea power. With their longships an superior seamanship they are able to launch surprise hit and run raids against Brettonia, Estalia, Ostland, and the elves with relative impunity. Eventually the Nords stop going a-Viking. Once the population of the southern provinces rebounded from the Chaos war, they were able to build a series of strong fortifications to keep the raiders at bay. They had more arable land, so they could field larger army reserves. Also, missionaries from the Imperial Church of Sigmar were able to convert a lot of the Chaos and Maan worshipping holdouts among the northerners and tame their lust for conquest somewhat.
South of Greater Norsca, Middland, once landlocked, now is not landlocked. Middland eventually merges with its closest neighbors including parts of what was Hocland, and Reikland. This new coastal nation is called Westland.
Ostland and Kislev merge into one kingdom, called Kislev because that is a cooler sounding name than Ostland. Ostland backed Kislev against the Orcs and Goblins and Kislev backed Ostland against the Norse raiders.
Tilea declares independence and forms a unified kingdom, but by this point their populations have almost entirely converted to the Imperial Church of Sigmar, so relations between Tilea and the southern breakaway nations from the Empire of Man are peaceful.
Two more nations coalesce out of the disintegrating Empire of Man. The nations of East Landlockia (encompassing most of Tablaecland and Ostermark) and West Landlockia (incorporating most of Averland, Wissenland, Stirland, and the Moot). Since they have no ocean access, they are not really important.
And the Elves
During the Great Chaos War, Chaos forms predominantly led by Slaaneshi forces simultaneously assault Ulthuan, Naggaroth, and Loren. All three nations drive off their attackers at great cost.
After the Great Chaos War, Khorne’s Curse weakens the power of all mortal wizards. Both Malekith and his mother Morathi, the twin rulers of Naggaroth, have relied on constant use of magic to keep the Grim Reaper at bay. They die shortly after the Great Chaos War. Both Malekith and Morathi encouraged their underlings to feud amongst themselves in order to prevent a large scale organized coup. With no clear successor to Malekith, the noble families of Naggaroth tore themselves apart.
Khorne's Curse also weakened the magic that prevented Ulthuan from sinking into the sea, but it was sinking on a slow enough timescale that the High Elves were able to make an orderly evacuation saving nearly all of their people and a good portion of their wealth. The High Elves sailed to Naggaroth and conquered their former rivals fairly easily. They re-named Naggaroth New Ulthuan.
Conquering Naggaroth was easy, but holding it was not. There were several deep schisms. Obviously the descendants of the Dark Elves and the descendants of the High Elves resented each other. Both the High Elves and the Dark Elves valued the Temple of Assurayan but sadly the temple of Assurayan was now underwater. A second schism arose between those who wanted to restore the tradition of the Phoenix Emperor and reformers who wanted to create a new system. Eventually the reformers won and divisions between former Dark Elves and former High Elves lessened over successive generations. New Ulthuan was ruled by a hereditary king. No longer did a magic ritual determine who ruled.
When Brettonia seceded from the Empire of Man, the Wood Elves saw this as the opportunity to drive both the Brettonia and the Empire away from their beloved forest. This made the Wood Elves hated by all the local humans. Then they had to face an attack by Orcs and Goblins.
Beset on all sides, and still reeling from the Chaos war, most Wood Elves only had three options. Merge with the spirits of the forest and cease to be elves or seek refuge in New Ulthuan and cease to be a part of the forests. Or they could die. Most died.