I think scenarios are a really good idea. The problem with OP1 in this respect is that it is a lot like sim city. You pretty much start from the same point at the beginning of every game and there's no specific goal. Sim Cities do include scenarios though, but they are stand alone type of scenarios that gave you a starting city and specific goals. (as opposed to OP2 which is story line driven and each mission furthers that story).
Since there is no driving story for OP1, I suppose you could create one. But I think I like the stand alone type of scenario better for OP1. With that, someone could still create a continous story if they wanted to by having several related stand alone scenarios.
On the other hand, scenarios might be a bad idea. Why?: They tell you what to do with your game. If you don't want to do that, it's too bad.
I think I like to play OP1 and Sim City (and Civ, etc...) because in the end, I have a civilization that I built. Not just another completed level. Of course, there's no reason why you can't have both ("sandbox" mode and scenarios).