It took me some thinking but I came up with it.
Could the footsteps at the end of Die Rise be a giant? Maybe Treyarch put those footsteps to hint towards a giant. We also hear what sounds like shackles or chains and then a gate. Doesn't that sound like a factory?
The giant could be the metaphoric giant spoken of in the description of Der Riese. The gate could mean that it's the end of the new 4's story...for now. In the torn piece of paper in the comic book, there seems to be what looks like an old video reel.
If it is a video reel, It could be just a part of the aftermath of Der Riese which could mean that the O4 could return. We may be able to play in the aftermath of Der Riese or a different part of Der Riese.
Could we return to Der Riese? Could the O4 return? Do we play as the new 4 to get the O4 out of the factory? Do we chase after the O4 as the new 4?
In time, we shall know. If anyone already made a topic like this, show me proof and I'll apoligize and let this thread die.