For those who haven’t seen the massive influx of threads about the new gameplay footage, briefly take a few minutes to view them.
As others have pointed this out on the forum, it could be the case that the multiplayer maps added are part of a much larger entity.
In this video, Beaver Creek Gameplay Footage, from 1:25 to 1:33, the camera pans towards the added teleporter cave engrained into the rock wall. It then starts panning upward, appearing to be an aperture in the ceiling of the cave; it appears there is a much larger area through this hole than one might expect. Further reinforcing this idea is the logical question of why an orifice would be in this otherwise enclosed area (obviously excluding the entrances).
People have also observed the skyboxes in the maps varying maps appear to be semblant between Beaver Creek, Hang 'Em High, and Timberland. I personally think maps like Beaver Creek and Timberland could be part of a larger Forge World, but Damnation and probably Hang 'Em High would be there own maps.
Frank O’ Connor has also further iterated that the maps in Halo: CEA will be significantly more Forge oriented than other maps (similar to how Tempest was a great map to Forge on).
Other pieces of the mystery surrounding what Halo: CEA and the TU will bring is here:
Powerful Pistol Evidence