Interesting connections, BHS. Personally, with all this information, I kinda have an idea. Let's see what we have here:
The "new" Der Riese loading screen
The "new" picture of a wall weapon
An area in Der Riese that looks very similar to the "new" picture
I think this somewhat hints at a possible remake of Der Riese that Treyarch had in mind during the BO2/Ghosts DLC season. Like I said previously, when the new personalization packs were announced during Ghosts' run, some people (including myself) hoped that this could mean that Treyarch would re-release older maps too. Now, let's just assume that they were in the process of remaking Der Riese, aka one of the most popular Zombie maps in history.
In the process of doing so, however, they decided to replace some of the wall weapons with newer weapons that fit the BO2 engine instead of remaking them ala the MP40 and STG. Now, you might be asking what weapon is depicted in the outline, then. Well, that's a bit of a tough question, but I think I found the best answer. By looking at the HUD icons for every gun featured in BO2, the one that matches the outline the most is the SMR:
Of course, the resemblance isn't perfect, as the barrel in the outline isn't as long as the SMR, but you cannot deny how much they look alike.
So all in all, I just believe that all of that information just lets us know that a Der Riese remake (and possibly others) were in the process of development before they were ultimately scrapped.