I've always found the concept of Luna being literally banished to live on the surface of the moon for 1000 years (instead of say, spiritually trapped/bound to it) to be equal parts silly and charming. There's no doubt that the lunar mare encourages some thoughts about ponies in space, but the more archaic nature of the show's setting (which I consistently prefer to the more modern elements that increasingly seem to be slipping into it) makes "astro ponies" feel like a anachronism to me. This is even after Dash dressed as one in-show. Still, who doesn't want to draw a pony with a fish-bowl over her head? So I can't deny the appeal (clearly).
As a result, my favorites are the ones like the aforementioned fanfic that take their cues from the earliest science fiction, where the science was mixed with a hefty dose of the fantastic as authors imagined crazy civilizations and methods for visiting them. As I nod, I designed Applejack with her own impractical source of air… oxygen-generating plants in a bottle, likely fed by her Earth Pony magic. I admit, I may have also thrown some JJ Abrams-appropriate lens flare in there for effect too.