There's been a lot of speculation regarding the possible reasons for Rockstar's decision to postpone the release of Grand Theft Auto IV from this month to some time early next year, but recent reports point to a possible focus on increased online functionality as the main cause.
The developers were considering the possibility of having an open world completely unlocked from the start of the game. Rockstar is aiming for at least 16 simultaneous players, with more than just usual deathmatch modes.”
Grand Theft Auto has strayed online before, most notably in the Stories iterations of the franchise on PSP. And whilst nothing has been confirmed as of yet, the developer has previously stated its intention to take the series in a brand new direction.