Yes..... the game doesn't end. It's open-ended =)
After you are done with all agency missions (cool ending, too), you will enter what is called the Mercenary Mode, at which point your only goal is to reach the 100% of the entire game, including faction missions and takeovers.
You will then find yourself (at least it happened to me) making your own goals, such as:
- I will take over all airports
- I will have full stars on all weapons and vehicles
- I will fly to the highest mountain and parachute down
- I will drive a fast car off a cliff
.......... It's never ending.