This fork could be, articles as they appear it Wikipedia, plus other articles that weren't accepted. However, what about deletionists affecting the edits of existing articles? Swiping Wikipedia's history, and keeping your articles in line with that would be a serious pain.
Wikipedia could easily set a flag that says how core the article is and then users could select the level of inclusion or notoriety they want to see. It'd also be awesome if on every page they included some sort of visualization of editing activity & population of editors and the amount of viewers who have looked at the article in relation to the edits. This way a user can get a feeling of how many eyeballs have been on the page.
Trying to run a site that's exactly Wikipedia/MediaWiki, but inclusive, could be hard -- given the preexistence of Wikipedia, and the difficulties of attracting a critical mass to an almost-the-same project. Such a simple fork might be stuck as a sickly sibling for a long time.
Instead, I believe the inclusionist alternative will have to change some of the core dynamics of Wikipedia -- solving some of the tensions that have motivated deletionism in other ways, while creating out a distinct competitive identity. I have some ideas, which I'll be writing more about soon. But a major theme is: there's plenty of room at the bottom.
