I wouldn't mind seeing stillwater return, through a mod or DLC, don't get me wrong, but I just don't think it will happen. Even with the new tools. Just because a small section of a city exists, doesn't mean they can just rebuild the whole city. You need new textures or else it would just feel like Steelport with a different layout, you need to program the AI to use the roads correctly, you need to create a whole new city with collission buildings or however you call it where you can land on every rooftop.. Not just something you can do with Kinzie's toolbox, me thinks. It's not like Simcity or something. And then you'd have to create the buildings you can enter and design the whole interior.
What they could do is reskin Steelport. Make the buildings different, make the streets cleaner, I dunno. Make it look like a real shithole, or the most modern looking city ever. That would probably be possible as the city itself already exists.
I really hope it will be possible to recreate Stillwater, but something inside my head doubts it.