What occurs to me reading this thread, is that what would most please people is a scenario editor so that the community can make up all sorts of insane scenarios and situations, so that Toady can offload all the content-creation and work on other things. It wouldn't be a scenario editor in the conventional sense, it would be more about loose goals or conditions that can occur in any generated world, even irrespective of modifications to creature and entity raw files.
To an extent it could be said that such things are already possible, we already have people sharing interesting sites and discussing what to do with them, nothing stopping people from sharing saves and making it a challenge-game sort of thing, and of course we've got the popular bloodlines-style games.
The way I see it we're going to need something like this sooner or later anyway. It seems to me that Toady has a keen interest in encouraging modders, and tools to allow modding of things that are currently un-moddable would naturally lead to user-made scenarios. Things like the sort of settlements that a given entity / creature would live in.
For example, people could finally make 'Orc' settlements to their taste, with crude magma smelting in open pits, all based around a tower where an evil wizard rules with an iron fist. Or an antmen city with warrens of bizarre yet organized tunnels.
I don't think we'd be well able to make stuff like this by typing into text files (it'd be rather too difficult for most of us to figure out), so it would require some sort of local map editor.
Once we have the means to determine what sort of settlement a custom entity lives in, we'd naturally want to tweak the rules of how they would operate in fortress mode, and would probably want the option to reside in these 'random' custom settlements in generated worlds. That's how, to my mind, it would naturally lead to a scenario editor.