I've read a lot about grammar this year, and nothing seems to fit with this usage.
Is it an abbreviation of some fuller phrase from the past? Many times if one can find the origins of a phrase one can understand its current usage.
I read another post here, that talked about differences between что новое и что нового. But nobody explained why the genitive is used. In my opinion there is no good reason, based on most of the grammar rules, unless, it was part of some fuller phrase. Or unless it's saying something like what is new AROUND you, in which case there is a missing предлог or an implied one like (около или у или...)