You guys are committing to making a lot of setting for this. Can I suggest something smaller?
One project that I started and never finished (and I’m not suggesting you use the project, just the structure) is something I call “Six days in Rhukaan Draal”. The ruler of a city has left for some purpose and left a weak lieutenant in charge. Soon after, there is an assassination attempt on a popular figure.
Whether the attempt fails or succeeds, several factions blame each other for the assassination attempt, and over the course of days tensions rise and eventually they start staging raids on each other and the city divides into armed camps. Each day the DM randomly selects factions to take actions against other factions, and the chaos level in the city increases, sort of like the Chaos Index in Fever Dreaming Marlinko. Depending on what the players do they can increase or decrease the chaos level.
There are three defined potential adventures. There is an investigation adventure where the PCs try to unravel the assassination plot. There is a lair assault where they are recruited for a lair assault/heist/rescue against one of the factions (slavers in my campaign, with a lair that looked a lot like A1 for reasons that will become clear). Or they can ignore everything going on around them and explore a dungeon below the city. Trying to keep the peace, or siding with one of the factions (all of which will be trying to recruit the PCs if they gain any notoriety), are other options.
Also, it starts to rain. Once enough actions are taken against one of the factions, the sewers back up; eventually this will flood both the dungeon and the slave pits, which makes those adventures time limited. And eventually the city’s ruler will return and restore order, making the other adventures time limited.
So you have a city sandbox with a bunch of potential adventures, none of which have to be that big because of the time limits. They range from investigations to intrigue to investigation to dungeon crawl to heist/rescue/lair assault (depending on the players’ approach to the slaver compound). With a potential social component because of the factions. And the city changes over time, both because of the rising tensions between the factions, and the rising floodwaters in the slave pits and the dungeon.
And instead of a city you could use a region. And instead of factions you could use a number of bordering fiefs, with an absent overlord. And instead of a flood you could use a famine, as conscripted peasants are not tending crops. For that matter, the rising tensions could draw the region closer to The Plane of Eternal War, and bizarre and violent phenomena could bleed into the world.
And one of the adventures could include a hexcrawl through the wilderness, and another could include a pointcrawl amongst farms, villages, castles and other landmarks. For that matter, you could introduce some of the elements of domain play.
And by the time the overlord returns to restore order, the players will be familiar with this seething mess of a sandbox and a lot of the people in it. And if their actions please the overlord, perhaps one will receive a small manor. While their jealous neighbours plot against them.
Well, at least the maps would be smaller.
Also, has anyone else ever noticed that they tend to DM the game they wish they were playing in?