The phrase "inherent assets" goes straight to the heart of the matter. NWN is a medieval fantasy game, and the owners of that game like it that way. They will not wish to alter their games, and download such huge haks, just to try a post-apocalyptic module. A separate game would allow for a more cohesive and targeted game.
When I dled the d20mm system and installed the custom content it completely altered my NWN install. Files in the Override folder, .tlk files, 2da's, haks, altered load screens in the game, and GUI appearances. Also, when I worked on my own custom mods there was problems. Even after I removed those custom content the game was still messed up and I had to reinstall it. I am not saying that a better system than the d20mm stuff couldn't be devised. It would definitely be a more reliable system if it was released by a professional design team.
Perhaps the answer is the advent of a universal player that allowed you to buy add-ons. For example the basic universal player comes with D&D stuff, but you could buy post-apocalyptic content disks, or Star Wars content, WWII content, Civil War content, extraterrestrial content, or to add your own custom content. Like the ultimate, expanded Bard's Tale Construction Set. That would be cool, too.
This thread was a targeted question at the staff of Obsidian. I have played their past titles and I think they could make something truly great. With the addition of an editor they would be taking it in a new direction. Console games are eating up the market, shelf space for PC titles is shrinking at your local retailers daily. Playstation 3 is a computer with no mouse or keyboard that links to your TV. Modding and Custom Content are the defining feature of computer gaming. I think it should be spotlighted.
Just some ideas.
As for me, I have spent the last four years learning how to script and write dialogues, make maps and objects, and all the other myriad details that need minding while making mods. After seeing the work of the Community Expansion Pack (CEP), Dragon Lance Adventures (DLA), and recently the d20 Modern Mod (d20mm) my interest in custom content has been sparked. I have nearly finished the gmax tutorials and also the NWmax tutorials and I am learning modeling and animation in anticipation of being able to create my own custom content. Alas, I am only a beginner. Making custom classes and races is also a difficult process, and editing 2da's is new to me.
But I will learn.