I wouldn't mind the moon idea, maybe make it froob accessible...
And not all aliens, some space mutants too, maybe a few lost explorers/ colonist that have gotten lost on there way to Rubi-Ka and they just went mad. I think that would be pretty cool.
Maybe make it with different playfeilds, because normally, there isn't really a good place to go to after totw, except foreman's but thats only good for exp til' bloaters are grey to you and after that, its good for loot. So maybe make them level locked, based on your level, say level 61-75 can enter one dungeon, level 76-85 a different dungeon, 86-110 another dungeon, etc.
And please, make it decent exp, say like 20k-50k exp, depending on the level of the mobs. So its better than missions, but not as good as the borgs.
Also, of course, have glass windows, or a dome, showing, Rubi-Ka and possibly ships coming and going from the moon.