A lot of people are speculating that the next map will be Agartha, but were we already at Agartha on Shangri La and Mob of the Dead?
We don't really know much of Agartha from the game, so like other historical and religious things the game has previously used, the game's story line puts a spin on what the real thing is to better suit its needs. The main things we know about Agartha from the game are that Maxis has been obsessed with it and that Shangri La was supposed to be a gateway to it. And like the Aether, its somehow connected to the Vril. So was the eclipse Time Travel portion of Shangri La really just us entering Agartha? And is Agartha just a dimension that is created by something or someone's perception of the real world, much like the prison on MotD, where multiple religious themes about the afterlife come together.
In Shangri La, we know Brock and Gary entered the time warp before they died. Yet once in the past, they are trapped in a loop and keep reliving the moment over and over, kind of like the Mob of the Dead crew who keep reliving their version of hell over and over. So maybe Agartha is some kind of afterlife realm for the dead controlled by what's in the Aether, and Brock and Gary were stuck in it at that location. On Shangri La, we are able to exit the time warp, as do Brock and Gary when we go back to regular time. So it appears we pull them out of wherever they are. If Shangri La is indeed a gateway to Agartha, then it would make sense to pull things in and out of there and manipulate both places. It also makes sense that Samantha is still controlling the zombies trying to kill us in both times since she is in the Aether and can go after us in both real time and in the other dimension. If it was real time travel which took the crew to before the meteor hit the pyramid, then there would be a good chance Sam wasn't in control.
This could also explain Maxis' main goal of reaching Agartha in the first place, which is something he wanted to do ever since Group 935. He says in one of the early radio files on Der Riese that he took the lose of his wife hard and couldn't stand to lose Samantha. So maybe he was intending on entering Agartha and bringing back his wife. We know the MPD on Moon when powered up, is a gateway to the Aether and it collects souls. So maybe Argatha is actually in the Earth like it is fabled to be and it also collects souls, so Maxis wife would be there just like the MotD crew. But Maxis was shot on the Moon and the MPD wasn't powered up yet. So his soul would still be loose which is how he communicates with us now. Spoiler At the end of the Buried EE, he explains he now controls the Aether, so it sounds like he can also control Agartha. He then says everyone will die and he will be reunited with Sam. So if he enters Agartha, he can shape it however he wants, much like how the higher power on MotD tortures the prisoners, and be with his Wife and Daughter.
So if any of this is the case, the next map could be Agartha and still be any destination Treyarch wanted. The new team could enter the Rift at Buried and enter Agartha, but the map could be a giant version of Der Reise with bodies everywhere like MotD if thats what Treyarch wanted. At the same time, the old crew was on Moon still. In the radios when Richtofen first went to the Moon, he was also teleported to a jungle, probably Shangri La. So the old team could easily also be teleported back to Shangri La and reenter Agartha from there. So both crews could be on the next map or maps. If they were to bring some old maps back, they could create them as Agartha maps and have bodies everywhere and other interesting things added like the Dog Heads or other mythical things to make the maps different enough from the originals.
Another thing to consider is that maybe the Earth itself is already merging with Agartha through the Rift. The two dimensions are becoming one which is why the Earth won't make it according to Maxis, but Agartha will. This could explain a lot of the mysteries of the story like why Nacht is on Tranzit, why an old West style town is in Africa with ghosts, why Die Rise seems to be the only map that for sure takes place in the future, and why the players keep reliving their own loops each time they die, much like Gary and Brock and the MotD crew. So if the Rift isn't fulley repaired, we could get a hell on Earth type storyline, which would be an interesting starting point for the next game.