Ok, let's see if this works. In the interests of building bridges instead of burning them, let me outline how I would run my next Sandbox campaign, which won't be for some time as I've just embarked on my current one, but, at some point in the future, this is what I would do to run my next sandbox. Buckle up boys and girls, this might be a bit long.
Step 1. Choosing a map.
Ok, here this is easy. The unbelievably talented
@Dyson Logos has recently being doing a project he calls
The autumn lands I both love
@Dyson Logos and hate him for being so damn talented at the same time.

Anyway, I'd use this map:
View attachment 404413
It's freaking GORGEOUS. Lots of goodies. Tons of stuff to play with. I have zero idea what the actual story of this land is, and don't care. I'm just unabashedly stealing this map.
Step 2. Creating the Truths of the World.
Now, this is where system steps in. I am a drooling fanbois of Ironsworn and I'd probably go out and get Delve and maybe a couple of other books as well. But, when playing Ironsworn, the basic Session 0 is used to establish the truths of the world. In the Ironsworn world, the idea is that the land you are in now has only recently been settled (think Vikings in Vinland or Canada). That gives a kind of base to work from. Now, as a group, we decide the Truths of the World which more or less sets the boundaries of the genre - is it a high fantasy world with wizards and whatnot or is it more or less historical fiction with no magic at all? Are there monsters in the world? If so, how common are they? Are there other species a la demihumans in the world? So on and so forth. This is done as a group exercise so everyone gets a say. IME, it generally works out with ties simply being put to a vote. So, that's the background of the world taken care of.
Step 3. Choose a Starting Point
This part is fairly easy. Find a town that people like the name of and that's where they are going to start. Even easier since nothing on this map has names.

But, if you look closely at the different maps, there are villages and whatnot scattered around. Find a couple people think might be fun - near the mountains, near the water, near the swamps, whatever. And, at that point, we're good to go.
Huh. This wasn't quite as long as I thought. Players create characters and I take a few minutes to bang out a town of some sort. Maybe consult the Oracle to get an idea or two for a potential conflict that needs resolving. Or not. The party could simply Undertake a Journey to go to somewhere on the map and I know that the system will build on stuff as we go, developing into an interesting campaign. The players are meant to each start with 2 (3? Sorry, book is not in front of me and I could be misremembering) Bonds with someone, so, right there, if I've got 4 players, I've got 8 NPC's with direct ties to the PC's before I even start. Additionally, every character starts with an Epic Quest of some sort, something that takes a VERY long time to resolve which also drives things along.
Oh, I guess there should be a step 4.
Step 4. Keeping meticulous notes.
This is very important. After all, stuff that gets generated in play can potentially be important, even if it's not important now. Keeping notes means that I, or any of the players, can call back to earlier stuff to drive events.
So, there. Done. I'm ready to play my new sandbox. With a map like this, I should be able to keep the game going for a year or more.