forward thinkers in Brooklyn (where else?) have been doing just that, brainstorming possible uses for the vasty public(!) space under the BQE. They even brought in the Space Buster (below), a sort of R. Buckminster Fuller-by-way-of-Rube Goldberg contraption to encourage creative thinking.
The most popular potential use seems to be community garden space. I'd like to see a kind of boardwalk stretching for miles--or at least many blocks--with art, cafes, and beer gardens. It might be a bit loud but just imagine the views of the city from ground level!
Bring on the trolls!