When you united and conquered Calradia, the map will change to a world view where there are 7 other kingdoms the size of yours and you get to travel via sea towards the west and north and via land towards the east and south to new lands and try to do a world domination. Because the map is so huge, you might not be able to conquer the world in one lifetime, hence make sure you groom your heirs well so that you have someone to succeed you.
Some endgame content will include investing in new tech and new weaponry (smithing), horse breeding (horse riding), new fighting style/stance (various weapon skills), also expect internal warfare between your vassals (steward, charm, roguery) where you have to get involve to rule over them. Last but not least, rumor has it that an ancient civilization lies hidden where one can find OP raw materials to craft the legendary siege/weapon/armor. Expect Knighthood / Bandit orders to form and surface over your dominance of 10-30 years where it will challenge your kingship. Keep your allies close and your enemies closer because one mis-step and you might have to restart the campaign in Calradia as your son/daughter with everything lost.