I would second what junfanbl says and admit that I am doing an open world map for my first game. I can share some thoughts:
So far I’ve gone through many iterations of a base map in World Creator until I was happy enough to bring it into Unreal Engine. I would say now, don’t try so hard to get it perfect first. I have had to do pretty major changes in UE. Once I had the terrain settled, materials, roads, foliage, etc. (Brushify stuff mostly) it became to start looking into locations locally and I realized that my map was much larger in reality than my imagined relative locations in my head. For example, I had made an area for a cabin on the top of a mountain. When I got around to placing an actual cabin and looking at scale by placing some vehicles and character meshes on the map, I realized that my house was sitting in a several football field sized area.
I added in foliage and filled out the areas, but then local locations seemed really far apart with not much player interest between them. The game I am building is closer to a Far Cry in game play, but the part about it and Breath of the Wild that I keep in my head is keeping the world interesting and alive. Random encounters, etc. I’m still doing it, but I had to put my work PM hat on and start documenting what’s where, why it’s there, how it exists, what it’s short history is, etc. I’m about 60 pages into a spec answering those kinds of questions. It turns out that it’s really easy to buy great looking assets, and you can assemble beautiful stuff, but it needs to make sense. So as I work through it and add locations, I have to go back and retrofit other locations to address logical holes they create in either story line or just common sense.
Kind of off topic I guess, but I would I guess add on to what he said, that building a big world is not just about can UE handle the assets. (it totally can) but it’s about building local locations and areas of interest that don’t make it seem like you put the player in an empty world. Also on the locations, it took me a bit of effort to not think of them only in the style I would play them.
It’s totally doable, but I would also recommend a beefy machine. I had to upgrade to keep from taking lots of breaks for shader rendering.