Mod Mark said that the team also considers what it's focused on recently, what areas haven't been touched on in a long time, and what parts of the game may have become dated. Content additions and improvements are then scheduled according to the time available to the team.
One lesson to take away is that it always takes the development team a lot more time and effort to create new content than players assume. Sometimes even a single quest may spend up to a year in development before being pushed to live.
You can watch the full dev diary after the jump!