Perhaps the best example of a modding community is the one revolving around TES III, TES IV, Fallout 3 and Fallout NV games. That community is up and STILL running. Take a look at what these games have become thanks to the modding.
Nehrim and other adventures are also a fine example of how a toolkit can make such a huge difference.
Voice acting is not a problem, IMHO. 99% of the mods do not need/contain VO and they are still magnificent.
I also consider that TW2, in it's now attained high professional standard, will feature a similar SDK, thus I can only see it as extensive and complete as the Construction Kit for Oblivion. I really don't think there is another choice for CDP than this, honestly. And this is a good thing. :)
A big reason for the success and longevity of Bethesda's moddding community besides the construction set, is their assets can be easily manipulated with open source tools, unlike Witcher's assets. The textures alone, Witcher's textures come in some malformed xbitmap format (and I call them malformed because I can't open them in Xwindows or anything X ), which have to be converted to dds, and the dds files can only be edited using the nVidia plug in for Adobe Photoshop, otherwise they don't work in-game.
Same goes for the meshes. I know someone wrote an import filter for Blender, but there is no export filter yet. There isn't anything like nifSkope either, where you can see in real time changes to textures, and you can even do mesh modifications, rigging, etc...
I see your point. But in my opinion that's not such a major drawback. TW2 has already some great PC specific assets (unlike Oblivion or Fallout games - for example low texture resolution).
But, even with some limited features, it would still be possible to use the existing assets (which are looking to be quite interesting) and create something quite fresh/new. There are some very talented people out there that can make great things almost out of nothing.
The only thing that would limit one to create new and fresh content would be another representation in the likes of the D'Jinni.
Besides, the modding of TW2 doesn't have to be exactly in the same genre of TES games, with thousands of armors, items, companions, weapons etc. One could create a new adventure, a new quest hub, some new features in-game, new camera style for new gameplay modes - diablo-like ones perhaps... so on and so forth... Look at StarCraft 2: it's a RTS with third-person RPG mods. :)