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Open nonlinear

Some nonlinear games open up their structure and let the players loose. They provide a very wide possibility space and make it nearly impossible to predict how the player will reach the end. Such titles still come with a defined set of winning and losing states, and often a carefully crafted main storyline, but any attempts at plotting the player journey are futile.

In such games, players are set free to roam and explore the content and gameplay systems, making each playthrough unique. Some series, such as Fallout, Elder Scrolls, Just Cause, or Grand Theft Auto, do it by providing an open world and filling it full of optional activities and missions that can immerse their players for hundreds of hours. Others, such as Civilization and the Total War series, make their players miss on sleep by offering deep, interconnected systems and simulations with countless moving parts. Another worthy example would be the interactive fiction 80 Days, which is a unique take on an open nonlinear game with amazing, highly adaptive storytelling and a definite ending:

A game with a defined beginning and ending, but a wide and unpredictable possibility space

There are no absolutes in game design; it's perfectly possible to retain a level of control over the structure and story of the game, while still allowing for free-form exploration. Most open-world games assume a hybrid form. They do not want players to be overwhelmed or venture too far too soon. In order to do that, you can create a set of artificial requirements that players have to meet in order to access an additional portion of the game. This unlock mechanism will let you pace the experience better and let your players focus on a more manageable amount of content:

Rather than opening your structure fully, you can divide the game into a series of hubs with players progressing through them in a controllable manner

The scope implications of an open world game can be huge, making tools for content creation and testing of paramount importance. You'll need to create a lot of content to make the world feel interesting, often with a risk of having to throw it all away as you modify and polish your gameplay mechanics. And as always, without solid second-to-second gameplay, no amount of content will save your game!

There’s always an option of going after a wide possibility space through the depth of gameplay. In such case, you'll require countless hours of prototyping, playtesting, and bug fixing to ensure the system is fun and performing as intended. Deep systems and simulations can be very hard to create and especially hard to balance, as a single change can have unforeseen repercussions across the entire long-term playthrough.