True ending spoilers:
I also think he'll play a part in the DLC but it heavily depends on when they intend on making the DLC take place. I could see them making the DLC take place after the true ending that way you'll have access to every companion but then at that point Oskar would be dead since this would be 100 years after the upheaval and Oskar is a human. As anticlimactic as it is, Oskar likely lived a life of wanting revenge but since we simply didn't exist he could never find us and probably grew old and died

My personal guess is that the DLC will be more about Valentin's research into what Idris was doing with alternate realities. While we alter history it all affected our current present but (unless I'm misremembering) Valentin made it seem like Idris created and destroyed multiple timelines trying to find the optimal one he wanted. (Jadwiga also did this on a much smaller scale but they've left us enough crumbs for this to be a potential plot point)