The Halo of Earth is what man has placed in orbit around the planet, themillions of objects, big and small, that whirl around the planet. When mankind suddenly vanishes this vast repository of technology suddenly becomes a junkyard, an ideal place for a mechanoid! Of course a lot of that stuff isnt in a healthy state, it might be junk left there from times gone by, it might be in a decaying orbit and will plunge back into the atmosphere anytime, it might have suffered damage due to neglect or impact.
The only problem with a Halo scenario is enemies? Who or what are the things you are going to be fighting. It wont be animals or flesh monsters, how would they have got there and how would they survive. So we have to introduce an alien enemy, one that can survive in a limited environment and will pose a threat to our mechanoids for some reason. You can always use other mechs, but its nice to have some variety.
Space Wraiths: nanite life forms based on energy who have moved in after all the organic life vanished. They might be a mutant remnant of whatever destroyed the human race, now adapted to life off the mechanoids.
Martians: who may or may not have been responsible for the human race vanishing, but now they are moving in and stealing all the neat hardware. Or they have arrived in motherships and have conquered the human race and are slowly shipping them back home in grocery bags.
Humans: they didn’t all die out after all, the ones in the Halo seem to have survived whatever happened on the planet below.
Zombies: if you use survivor humans you can have zombies and a zombie plague, gotta love zombies (and they don't breath).
Predators: again if humans survive then alien predators will be seeking them out, maybe the planet below belongs to the predators now, but without the means to get into orbit… except for one!
Space Stations: big ones with lots of room and lots of uses - research stations, military stations, really secret stations.
Asteroid Habitats: large asteroids moved into orbit, mined for their minerals and then dug out to create habitats for people.
Elevators: huge lifts from the planet surface to a perfectly matched orbiting station, connected by a super strong cable (or cables). Makes lifting things up and dropping things down very efficient. Creates a connection between the Halo and the planet - something to aim for and guard against.
Lots of little places that you have to use to get from one place to another, and thus offer a neat enclosed adventure environment.
Something is coming up the Lift!
If we can get to the lift we can go down to the surface!
What happened to everyone?
A Predator is lose!
Zombies are coming!
A Martian invasion has arrived, the motherships are in orbit, humanity has abandoned the Halo, but someone has left you behind to complete a mission (cause the martians ignore mindless robots), one which would destroy all the motherships, except they died before passing on all the information…