I have a main hook in the campaign that's kind of a large background problem that impacts almost everything in some way, something that brings a lot of change. I had a ton of rough ideas for what would be in different regions of my campaign setting/map and conceptualized those as "quests", just a name to put on one cool idea/mystery and tease it players. At the start of the campaign there's an immediate hook and thing that happens which ties into the main large problem and teases a lot of its mystery, brings a lot of change and danger but they players don't really understand it full yet, and forces the players to want to do something or go somewhere. Then depending on which "quests" they seem to take an interest in, what hooks they bite into, that's where they go and I can detail the place they'll go to more.

For example, my current campaign :

• At the start, for one reason or another the players are present near a clash between two armies, a war has broken out between two minor kingdoms in a vast, wild and untamed region full of history and mystery (doesn't really matter why or how they are here, as usual the players understand that they need to bite into the original hook or there's no way to even bring their characters together). As the armies charge, the players have a strange kind of vision, on both sides stands a strange knight in golden, shiny armor, standing on a diseased horse. A golden mask covers his face, but flies betray the decay hiding under it. The knight screams of glory and victory, and charges. The armies clash. It's a bloodbath, and suddenly a sort of necrotic storm is felt, a sickening feeling felt by every living thing. The fallen start to rise, and both armies have to retreat as they start being massacred by a rising undead horde. This becomes known as the butchery of Iar'Limb. I'm skipping some bits but basically there's a bit of setup for things around them that have been and are happening, the kingdoms, the war, some history, etc. A big thing happens, the golden knight, the undead storm and the new army of the dead. The players attempt to help in whatever way they can but they know they have to run away too, they can't face this threat yet.

It's quickly clear that the entire region is affected in some way, everywhere rumors of magic acting up, spirits angered, planar rifts or undead energies, and somehow it all seems to be related. The general idea now is: explore and find stuff out. Some people might know more, some mysteries can be investigated to reveal a piece of the larger whole, some people might help. A lot of different factions and NPCs roam around and deal with their stuff, the local problems, the conflicts, their own goals, etc.

For example the place they start has a few "quests" I tease from the start : the old caravan that one of the players knows has a mystic that could know more about shadow plane/undead stuff. One of the players was a soldier of one of the two armies and has now effectively deserted, so they want to leave the area quickly, they head to a nearby town where a lot of refugees are already gathering, the whole region seems in turmoil. In the town they learn of demons on a nearby hill that are haunting and corrupting local inhabitants, a fisherwoman talks of dead or diseased fish from upriver, a mad and rambling man runs in the town square shouting about the town in the forest appearing and disappearing in-and-out of reality, and hands the players a weird piece of some broken clockwork machine, etc etc.

The only thing I had ahead of time was a rough idea of what the main hook is about, but even that not entirely. Some details and fun encounters to quick off the first session, a quick map of the nearby down and some NPCs. My hexmap at this point is just my campaign map with the hex grid, maybe a couple ideas jotted down here or there, or large groups of hexes like "this region is all about fey stuff" and "this region will be witches and spooky forest creatures", etc. Then I had a bunch of rumors, cool ideas and quests to tease, and maybe a little note about how it could tie back to the main plot, but nothing more, don't want to prep for nothing.

After that first session, since one of the NPCs they met is an old sailor that one of the players knew from a previous adventure, they chose to spend more time with him and look into his hooks/quests, which tied to the demon hill, so they went and did that. I then detailed the hexes between where they were and the demon hill, I don't like filler and the point of them is not to spend an eternity in each hex but to represent a fun voyage with interesting things to encounter and stumble upon. Even mundane things will always tell them a bit about the region, its history, tie to some local problem or quest, etc.

Next session they traveled in that direction to find the demon hill, found a few things here and there in the hexes that built up the world around them more, some colorful NPCs, some places that developed the history of the area a little more, or just cool fun stuff. I especially like stuff that they can make their own and not things that are "depleted" once encountered. Like a magical stone that seems to be able to absorb curses, which is tied to some settlements around it, has a little bit of history and lore and NPCs which could have quests about it, but most importantly the players mark that down on the map and say "we'll know to come back here if we have curse problems", and of course on demons hills they find cursed items.

This is getting really long so I'll stop, but basically I frontloaded a rough map, rough regions of hexes with a few cool ideas scattered around, a bunch of smaller plots and mysteries that tie back to a big long-term mystery that the players want to resolve, and then I just improvise and "zoom-in" a little bit on a specific region or patch of hexes when I know the players are going to go there.

The hexmap itself isn't a major dungeon in every hex, those are scattered evenly with a few per region which all tie together in some way. The hexes in between are a sort of "area of influence" around those major hexes, they tease the plot of the big hex, add some detail, immersion, color, or cool stuff to find. If they know they want to go from A to B and they know where B is, then the "exploring" part is less important and can become tedious, so I don't put much effort in it, I emphasize the wonders and tribulations of travel. Where it really shines is when they know something important is nearby but not exactly where, and they have to look around, interact with everything to learn more, and will thoroughly explore a small patch of hexes.

The hexmap serves as knowing where the players are, there's usually only one or a few "sites"/things per hex. It's also how I track where the factions and roaming NPCs are over time, or how the big problems roll through the region (like the army of undead slowly spreading through the region as the days advance, and the players hear rumors and news about it, and how it impacts the rest of the region), and when they travel or explore I have a simplified routine like how long it takes to cross a hex/why, what they automatically find or have to search for, or some check I roll to see if they have an encounter and what kind (not necessarily a fighty one), etc. It's more a prep and tracking tool, and helps to keep it all straight in my head. It seems to be working wonders because the players at this point fully assume that everywhere around them is coherent and prepared and there'll be cool stuff to find, they've very immersed and are eager to learn more about the places they explore or the mysteries they are trying to unravel.