According to developer Housemarque, a Resogun DLC is in production. A fan asked the studio about its future plans. They answered:
At the moment we are working on Resogun DLC and another PS4 project.
Resogun was the game I personally enjoyed the most on PlayStation 4, I’m stoked for any future content. We’ll be sure to let you know as soon we get any further information about either the DLC or the unannounced game.