How about a Wikipedia "documenting" expressions in various languages? I
mean, try googling around for "bring it on" (~542,000 matches), or "what
goes around comes around" (~98,500 matches)--and try finding out what
they /mean/. I'm pretty sure you won't be able to find any explanation
anywhere--I haven't.
Now I'm aware such a topic is more "dictionary-like" rather than
"encyclopedia-like"--I guess that's the first objection which comes to
mind, although we already have a Wiktionary--, but is Wikimedia's
purpose to only support encyclopedic projects, or to support projects
which would be helpful?
Also, think about the various languages which make use of
expressions--due to the human nature, I expect all languages do--, and
think how helpful such a (successful) project would be for people around
the world trying to find out information about this and that obscure
expression in some language. Bells and whistels included a la Wikipedia,
as in cross-referencing languages, historical periods, you know the
drill, I won't go into that.
What do you think?
--Gutza