I'm not completely sure which planet, but either Venus or Mars was theorized to have held vegetation at a point in time long ago; so it could be plausible that when you go to area 51, you also go back in time and to a different planet. The vegetation, therefore, would allow you to breathe oxygen whilst on that planet.
If you look on the wall near one of the teleporters in der riese, you will see a message that says something along the lines of "you must ascend to the Aether." So, in theory that could mean that the protagonists will go to a different dimension called "the Aether." I know it's not much to go off of, and you might think that if they were hinting on that, they would've put it into black ops; but, the first black ops map was originally intended for world at war. Maybe they were intending for "the Aether" to be the last map in black ops, but since the kino map got moved to black ops, it would have sub-sequentially pushed the "Aether" map to black ops 2 due to time restrictions, and wanting to put more plot into it, and seeing how they have a whole entire game full of maps to make, it doesn't really need to be the first, maybe it will be the second, after another adventure on the moon; on a different spot in this web-site, you will find, under the classified section, a picture that just says "TACTICUS"; in another picture, with a darkened view of some planes, there is a black spot in the back round, if you brighten the picture up a bit and over lay a map of a section of the moon, you will find they more or less match up, the black spot would match up around a specific section of the moon, a crater to be exact; And the name of that crater just happens to be, "TACTICUS". As I said before, Another moon map location perhaps?
Tbh. I also thought of dimension travel. Because of this reason, but also from the radios on Moon. I can't remember them in detail, since I didn't listen to them anymore for over months. Anyways, some of them also said sth. about dimension travel, I think it was some nut they have teleported into another dimension. And since the blew up the earth with 3 or 4 badass rockets, nothing much to do here anymore. So yeah, I bet they will travel into another dimension.
But if they do, the question is, what happens with Richtofen? As far as I got it, he and Samantha switched their souls into the others body. Means, Richtofen is in Samanthas body, which is on Moon. So... I dunno. It is possible that Moon will be continued with an advanced story, that will make all of them then travel through the dimensions. Maybe you will have to do another easter egg there, connected to Dr. Maxis, which he created in order to save Samanthas body, n make it travel to another dimension. And then they gotta travel too n yeh, maybe even meet that girl, who is not in the kitchen as you may have seen in the zombie artwork or she even appears on Moon n thats how I would explain to do it with 8 people, that she and her crew arrive there through dimension travel and the 8 of them do that easter egg which makes Samanthas body, therefore Richtofen, travel into another dimension, wich was originally planed by Maxis to keep Samantha away from Richtofen. And then the girl teaches them about dimension travel n bla n then, the dlc content begins.
Just my thoughts. Hope you can follow them haha