A couple thoughts on the outertale AU and I love the idea of magic in space but HAVE YOU CONSIDERED:
-Monsters and humans getting into a solar-system wide war. When the humans win, they leave them all on the moon and expect them all to die
-Through ingenious re-purposing of previously considered wrecked/unusable/rejected technology, Monsters are able to set up a stable but fragile self-sustaining ecosystem.
-Asgore needs Human DNA or something in order to re-activate the wrecked Human space vessels and get back to earth
-The fallen children are the survivors of space accidents or whatever the possibilities are nearly endless
-Sans has some pretty awesome external…. plasma blasters? Or something, that he can control from what is basically a futuristic Google Glass. With the compture in the headset, he can also MANIPULATE THE GRAVITY GENERATORS IN THE OUTERGROUND