In fact, let's take a look at two games that used dynamic content and see what we find there.
Fable II: While most of this game was the usual scripted missions, there was some dynamic content going on. This manifested as the way that the characters and world reacted to your choices. To cite one example, a given town could shift between lawful and lawless, prosperity and poverty, based on your actions there. That's cool on paper. But in reality, that kind of content isn't dynamic enough. If the player picks one general approach to play and sticks with it, he'll never even notice the town's subtle shifts because it keeps evolving in the same direction. Only if the player is deliberately whipsawing between good and evil, prolificacy and penury, will he see the town jump from lawless ghetto to well-guarded suburb and back again, and only then will he realize the scope of what the game might do. What's needed here is content that waxes and wanes due to factors other than just the player's journey. Set the baseline behavior as turmoil and churn and then leave it to the player to do the work to dampen the turmoil and push the town in a deliberate direction - and then work to hold it there.
Left 4 Dead: This game is really the poster child for dynamic content because that's its raison d'etre. Valve wanted to explore dynamic content in the form of their "AI Director". They built four short "campaigns" but then put the real work into a hugely varied and responsive system of spawns, spawn points, and spawn rates. The enemies ebb and flow in a dramatic fashion, and the same level played ten times plays differently each time. One playthrough may have an epic battle in a parking lot, and the very next time the parking lot is deserted but three medium-sized fights break out in the surrounding buildings. So Valve did great work here, but they were exploring dynamic content within a very narrow definition. The player's ability to consciously influence the dynamism is about nil. The joy of the game isn't from interacting with the dynamics; it's the simple joy of shooting zombies over and over again on the same maps without it getting repetitive.
Okay Mister Smartycontentpants, What's Your Bright Idea?
Let's not screw around. Let's redesign Grand Theft Auto IV without scripted missions. They call it a sandbox game, right? Let's play in it.
So here's my intent for this exercise. I want the player to drive the experience by setting out some very clear goals he can pursue and a diversity of methods for achieving those goals. Then I want to put some dynamic systems in place that will respond to his attempts to achieve those goals and add challenge, surprise, and depth.
In my version of GTA IV, Nico Bellic is out to make his fortune in his new country. We show the player a set of progress meters he can try to fill. These are called Wealth, Looks, Infamy, and Influence. Each one goes to 100% and there are thresholds at each 20% increment. Here's how they work.