There would be many more scenarios, except that the scenario ability which was in Dom2 is broken in Dom3. Most of the features which should be in scenario will not work for a downloaded file run in local mode. To get a scenario game you have to step majorly into server mode, which means that it might as well be an MP game as long as you have already gone to all that trouble.
In Dom2 I had a scenario where D.A.R.E. (the Deadly Atlantis Rlyeh Ermor alliance) was well settled and defended on a map challenging the players to be forced into an alliance of their own to defeat the threat. But in Dom3 that becomes almost worthless as a download and only reasonably possible if I run it from my server.
Here is a thread about what is and isnt in the scenario tag of Dom3
http://forum.shrapnelgames.com/showthread.php?t=43209
And here is a thread which tries to get around those limitations by various creative means
http://forum.shrapnelgames.com/showthread.php?t=39304
If you do decide to do a scenario style game Im totally willing to host it for you.