Eh...while I agree the jukebox has his colors (the diamond belt at the bottom of the jukebox a dead giveaway), I sincerely doubt Plastic Man is just hanging out in Metropolis disguised as a jukebox. That whole arena is FULL of homages to characters, events and extra fluff that spans the movies, various tv series and comic books. That Jukebox that is definitely not Plastic Man in disguise is just another homage among many others in that scene, and assuming that it alludes to him being future DLC or that the Jukebox can somehow turn into Plastic Man for a level transition is a real stretch (no pun intended).

The devs certainly didn't forget about Plastic Man. Indeed, he is shown in the latest issue of the Injustice 2 comics (#16)—reluctantly—joining Batman.