A few ideas I thunk'd of;
Have an aboveground-underground system so that there are several human bases, for missions, you could make it so you have to collapse a tunnel to slow down the terminators as the base evacuates
Also have most of the terminators T1's and then either have them progressively get harder, or you could have bosses like the T2 and T3.
have defensive areas and quest areas, like in one spot your quest could be to defend an outpost as the terminators march down the hall or across a junkyard
have maybe a dodge the nuke type thing in one area/quest where you have to get somewhere but there are flyers patroling tha air and if you get spotted by one they bomb you.
Have several battles where its you and the computer humans attacking the terminators at the same time.
Make sure you make the whole map (except certain areas) junkyard so that it looks as if the world got nuked, and if it did everything would be scrap so dont have clean fields and stuff.
A suggestion for the terrain type is badlands, you can make a good junkyard type map in that tileset.
Maybe have a maze where you have to escape before it gets gassed by an AI.
Thats just what I could think of off the top of my head so whatever, ya...

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Thanks for the Ideas! Plus I already have a trigger for the Aerial H/Ks the one you called, "Flyers" Plus I'm going to have a few Ground H/k tanks partoling around like on the movie! I know about that the whole world has been nuked by SkyNet. I seen all 3 movies!