One of the reasons why designing and implementing online games is so hard is because it’s hard to understand the consequences of each change. Many times, you have completely unintended consequences which can affect a game in ways the designers did not foresee.
In a single-player game, it’s easier to guess how a change in development will affect a player. You only have a single player to affect in most situations, and the game is often limited in what responses a player can make. Games that strive to be open-ended, like the Elder Scrolls games or Grand Theft Auto, you will often see more unintended features in the game.
When you add in multiple players, you have to consider how the players interact with each other: how they can affect each other and what happens when multiple players act at once upon the game system. On top of that, you have continuous changes to a game world in order for it not to feel completely static. Add in an open-ended world, and it becomes much harder to accurately predict what the consequences of change to the game world will be.
How can an online game designer deal with this complexity?