First I'd expand the list of awards for kingdom building. As in a reward for every 50 additional hexes, rewards for having 5 cities, 10 cities, X cities, having a full grid in 5 cities, 10 cities, X cities, having 2, 5, 10, X grids in a city.
Second, how do the players and the rest of the people learn of Choral's return? How does he proof his identity? That might provide some story ideas.
Looking at Carrion Crown, in the first book there are XP rewards for research. So having some rewards for the players researching Choral and his allies could provide XP in a somewhat different manner than they are used to.
During the research they could come upon a legend of a weapon forged against Choral. This could be the result of a project started by Restov, but not finished in time for the final battle, or it might have been present at the battle. It could also be an artifact that is somehow connected with Choral; maybe they find an ancient prophecy linking Choral's demise and the artifact. So hunting down the item could provide a high level dungeon.
As Choral is aware of Nyrissa, he from the beginning worries more about the player party than any army; he knows he can overpower any army, but doesn't want to slaughter so many of his future slaves. So he plans and plots to minimize casualties while taking out the only real threats directly. He might use demons, human assassins, or just about anything else. Maybe use an army of giants coming down from the mountains towards Varnhold as bait for the party; use several waves of giants and other stuff to wear down the party, before a hostile adventuring party from the rival guide comes in for the kill.
Unconnected, or maybe not, a host from the Worldwound breaks through the line of defenders and spreads chaos through the region. While they won't reach the players kingdom, their neighbors will be destroyed without help.
With the players taking over Pitax, they infringe on the River Kingdoms. Lots of possibility of intrigue there; maybe the players decide to make a land grab against some of the less friendly neighbors in the south before Choral returns? A few provocations from the neighbor (or a third party wishing for that war) may be enough to get a good number of sessions; and it should give the players the feeling that they are driving where the campaign is headed.
A team of explorers has worked in Vordekai's tomb for months and removed many artifacts of historic significance; while the pathfinder society is present and supporting the effort, it's headed by people loyal to the kingdom, and when they discover a new secret door, they immediately send word back - the PS isn't pleased, but after Vordekai also worried about what might be hidden there.
Has the party dealt with the tower on Candlemere island? If not, there's now a cult there, and they have woken something.
A messenger delivers a warded box (Warding Box minor artifact - no way to tell what's inside the box) to the party from an unknown party. Inside is the seal of Amatatsu (Jade Regent AP). Opening the box at all attracts the attention of the Oni, and an assault group teleports with orders to kill anyone present and steal the seal. Now the players may need to find the heir, return the seal, and get them on a journey to deal with the Jade Regent, if they ever wish for peace. They could maybe go themselves, but it'd mean abandoning everything they have built... you need to judge if the players would do that. It's intended as a constant hazard to the kingdom until they find the heir and send her with a well equipped party north; after a while, depending on how the other party traveled, the attacks stop and they get a good friend far away.
A mysterious plague sweeps through the kingdom. Even casual investigation shows magical involvement in the spread of the disease. Is this some kind of revenge from beyond the grave by Nyrissa, or something else?
A mage that's lived in the kingdom for years goes missing and his golems run amok. What happened to the mage, and where did he get that much adamantine?
During the clearing for a new city district ancient ruins are found. Historians connect them to the Whispering Tyrant. Is there an army of undead inside, as the panicked population thinks, or is this merely a burial site used by the Shining Crusade?
I hope you'll find some useful ideas in this brainstorming.
