One of the biggest features in modern games is multiplayer, either over the internet or on a local network. Xbox Live, for example, is one of the largest multiplayer networks, and games such as Halo and Battlefield have incredible communities that sometimes never even play the single player component of the game. For this reason, multiplayer is a valid and powerful concept that should be considered for your games, although it may not always be appropriate for what you are creating. If your game doesn't fit a competitive mode of multiplayer, consider a cooperative mode. If multiplayer doesn't feel like it matches the experience that you want to provide, leave it out. The important thing is to ensure you focus on ensuring quality and fun, rather than adding in features that might not work.
One thing that I've omitted so far is a user interface system. This is crucial to multiplayer gaming as you need a way for the user to choose the multiplayer option and...