I think a fork of Discourse to make a wiki would be a relatively finite amount of work. Far from trivial, but not impossible either, especially for a developer with the appropriate skill set (which excludes me, I’m a Mac developer, not a web developer). I’d guess a good developer could get something basic working in a few months and maybe a years worth of work to get something really solid and appropriately featured.
The problem, as always, with forking anything is you then lose all future benefit on the other side of the fork (Discourse does not gain from any benefits in the wiki that would be appropriate, and the wiki does not gain from the continuing work on Discourse). That’s why I started with “I hope that is on the development plan somewhere” - because a non-forked, shared source wiki would be a far better solution.
That said, a forked solution would be far better that the current alternatives IMHO, so I’d be happy to see either.