According to Psyonix_Jerad (i.e, someone called Jerad who works for Psyonix) on the Supersonic Acrobatic Rocket-Powered Battle Cars forum, Psyonix are working on some DLC for their brilliantly titled game, namely, a new arena and new features, as well as some bug fixes.

There's no date for the DLC yet, just the above information. Let's just hope it's soon, we like free stuff.