For Rocket League a new (paid) DLC has been released on Steam: the DC Super Heroes DLC Pack. It contains two Bat-cars and more than 20 customization items from the DC-Universe:

Experience the ultimate DC Comics crossover as two classic Bat(tle)-Cars from 2012’s The Dark Knight Rises and 1989’s Batman come to Rocket League! Also included: more than 20 customization items and player banners based on Superman, Wonder Woman, The Flash and other popular characters!