I've got a couple of ideas I'm toying with, both of them single-player mods and geared around specific chracter classes: one for a starting first-level Thief PC, set in a city, fairly-open-ended but with plot threads involving rival gangs and religious factions; and another built around a Ranger PC who takes a job as scout and tracker for a border fort.
I have mixed feelings about the editor, though. It seems pretty easy to use from the beta toolset, and the scripting possibilities look incredibly deep, but the extreme modularity of the building blocks might be an issue in terms of trying to create a unique setting. For instance, I really don't like that it doesn't allow for slopes in the wilderness, other than the basic "ramp". It seems strange to me that an editor otherwise so rich is so limited in this regard.
But I guess there's a lot of room for user-made toolset additions, and the possibility for "mod packs" put out by Bioware, so my crankiness may prove unfounded in the end.