I was thinking of this earlier today. I know people have wanted to have mission cards. The current game is not setup for that however, you could make a map based on mission cards. The way you would do that is specify several missions on the map and the bonus for having completed the mission. Hold all of SA and Asia is 1,000 armies. It makes you still take your turn but statistically you will win. Everyone that plays knows the missions so people will work together to keep each other from completing them.
The only downside is that a specific map would need to be made with mission bonuses in the XML.
What do you think?
UPDATE
To simulate a mission based game, a map would be made based on missions. The missions would be written on the map for all to see. Each player could choose any of the missions. Missions could be holing all of the capital cities, holing the two largest continents, etc) if a player achieves a certain mission, on their turn they will receive a CRAZY number of armies (1,000). That way on their turn they will be able to conquer the rest of the world. It would act like a mission but still play based on the way CC works now.