A lot goes into game design to make your environments both believable and playable. Gaming environments bend to the will of the story, rather than to the will of nature.
Many open world games have to think about both the level design and environmental design as a coherent whole. Atmospheric storytelling. It has to look beautiful and organic, but also engaging and push towards gaming objectives. Things to actually do.
Gaming design was a slightly more straightforward process in the older days. “Mechanical gameplay” is a term for a more curated game experience, where the creators are guiding the users in a very specific way through their world. “Reactionary gameplay” is more adaptable to what the player decides to do. With reactionary gameplay, two players playing the same game will have two different experiences.
While there are a bajillion examples of open world environment building (Skyrim, GTA, Zelda, Fallout, etc etc etc) let’s add an example not brought up a lot: Sly Cooper.
Now, Sly Cooper is actually considered a platform game, but many of the levels take place in the same 3D world. Sly Cooper is a raccoon thief who steals with a purpose (kinda like Robin Hood). The game is split into “episodes” which each take place in a different part of the world. In “Sly 2: Band of Thieves” the first episode is in Paris, France (much like the first game as well).
The Paris of the Sly Cooper video game world has been modified so that jumping across rooftops and sneaking through alleyways is much easier. The city appears endless, but there are cutoffs like rivers you can’t swim across. Walls that give the illusion of endless access. Once you hit a river, you’ll see a road that you’ll be psychologically drawn to that will lead you towards the playable areas of Paris again. The Paris buildings and roads are curved and designed with your playable character in mind.
Each “level” that takes place in an episode of the game is still in the same city environment, giving it an open world feel as you complete objectives. There are “points of interest” which you travel to in order to start objectives that gives it the open world feel.
With the future of game design, AI will undoubtedly be incorporated more. NPCs (non-playable characters) in gaming environments are still very awkward and not very smart. Advanced AI implementation would include intelligent AI strangers and randomized events to occur in the game.