This is an amazing idea, actually. I truly would like the community to actually become one, in a very real sense.
But the problem is the issues of copyrighted property.
Basically, everything you see in Doom, whether it be present in the IWAD or not, will be copyrighted. I'm not talking about the code, which is GPL, but intellectual properties such as themes, level names, plot, monsters, weapons.
We could pay homage to them, but not recreate them. Not without some kind of licensing, as if some Doomer will actually pay for it :p.
Basically, what this would entail, would be a group of people to firstly 'design' (In the sense of storyboarding, planning, and brainstorming) a theme for the IWAD. The there'd be concept sketches, followed by a whole mass of textures, sprites, music, sound effects, etc.
And then, there's the issue if they're too 'close'.
I know that there are many TC's and mods that may have done similar, but they all require the original IWAD. If we were to provide a cheap (read: free) alternative to the Doom/Doom2 IWAD, then this would be borderline piracy, technically. It's about the theft of ideas - a grey area, but it can still get you sued.
Basically, we'd have to agree on some completely different theme, etc. I'd be willing to join an initial 'designer's commitee', and probably map for the finished IWAD (check here to see examples of what I can do, map-wise.)
So, in theory this is truly a groundbreaking idea, a landmark for the community. But in practice, it will be a tough goal. We need people willing to participate.