I too voted for the Moon as a new zone (and maybe Mars was the other I think) but rather than just reusing the previously established villain groups, I'd suggest inventing new ones.
I do like the use of the Igneous, they feel very "Moony", but what if the Moon were the first staging area of a new alien invasion. Perhaps Mars seeks to succeeed where the Rikti failed. Enter the "Martian Menace". Mars is a world gripped by civil war between two powerful factions (good and Evil, heroic and villainous). Give them a very Retro 50's Sci-Fi look and then you can package the Moon, the "Undermoon" (domain of the Igneous), a space Station, and Mars all in one Issue. You could easily justify another joint COH/COV mission area since it'd be a threat to the Earth as a whole.
My other advice would be a string of Timetravel missions (prehistoric, ancient greece, WWII, and a post apocalyptic future) but that's another post all together.
Capt. Glutton 39 Inv./SS tanker (Leader of the Deadly Sinners, Triumph)
