The common theme here is one of a disarrayed battlefield - destroyed and rendered a complete mess by the Legion in their preparations for the attack. No doubt caused by the onslaught of the Alpha Legion's agents before they even got there. Moreover, it follows the precedent set by the scenic resin base that came with Armillus Dynat.
Although I originally thought about a different thematic base for my Alpha Legion Unbroken Chain army, I can certainly dig the devastation theme. The benefit of it is that it allows a whole range of different terrain whilst simultaneously being able to cater toward different styles, such as variations in height as used in these bases.
This is exemplified in the upper right base wherein there is a wreck of a tyre adjacent to come cork, sand, slate and a few other bits. If you examine the base closely, you will see a small paperclip pin jutting out from the top of the tyre. This is where the leg of a marine will eventually attach to the base for a running, aggressive pose. For my snipers, they will serve as a elevated levels for the marines to take their shots from.