What kind of “future development” aspects have you included in your campaign world? My map holds a couple areas ripe for the lord’s “go build me a castle there” future domain building quests, but I’d love to be able to pepper in more elements that will grow as the PCs do.
Does your game setting have any rival adventurers destined to be rival barons? Political situations in neighboring kingdoms that your PCs will have to keep an eye on as they quest? What kind of rumors are you planting to coax PCs to where you want them? Ancient secrets to be unearthed over a dozen levels?
I plan the next session, with seeds and other potential things in an unformed state that can be picked up or not a we go. It's highly likely (given where one of the PCs is driving) that there'll be a war with Carthage, possibly with one of the Celto-Ligurian tribes nearby, too.
No rival adventurers, but there aren't really adventurers in my game anyway. There's plenty of other mercenaries, though I doubt any of them in the region have the PCs sort of ambition. Still not sure whether they'll eventually want to take over the city, or found something of their own somewhere.
They're held in place at the moment by a contract to an employer to train the city troops; once that is lapsed I'm hoping they'll be so deeply enmeshed in the local politics and their own schemes that they won't want to leave.
I have a general idea of the conflict, and I intend to write up a timeline of future events (that gets more vague the further out). But I have the advantage of writing up the whole thing before I run it, so I don’t have to worry much about time constraints.
Wish there was an edit button. To answer the OPs actual question, I want to have an idea of what sort of conflicts the PCs will run into around 13th/14th level, so I can foreshadow things, but nothing that far out will be very detailed, and certainly not set in stone.
My ACKS campaigns follow a particular methodology.
The sandbox is built ahead of time, more-or-less following the guidelines in ACKS. I do not necessarily flesh out every dungeon in advance, but I at least know it's rough denizens, size, and monsters, or what module I'll be using for it if I'm borrowing a pre-existing element.
I then set the setting in a state of chaos. Generally, in the Auran Empire, it's the declining state of civil government (pre-civil war, more or less), ringed with rising powers seeking to take what the Empre has. I sketch out the major players in the civil government and nearest enemy nation at a high level.
I then usually have a hidden existential threat. I sketch out a timeline in which this threat's plans will be operational.
Every few weeks, I update my documents on the plans of the civil rulers and existential threats, and factor in how they are responding to changing events.
Actual dungeons I try to have written 2-3 weeks in advance.
…And then your players accidentally release a powerful chaotic being every other session shifting the balance of power in the world. I know your Opelenean Nights group has been getting better about it, but for a while there I was wondering how long it would take for them to RUN OUT of elder evils to unleash/aggravate. Gotta love seeing what players do when you present them with a hornets’ nest and tell them it’s filled with gold… and death, but also gold!