An earthbound genre series will often look to supernatural and paranormal stories as a source of plot ideas. And there are plenty of options, from pop culture icons to obscure local legends.
Doctor Who and its spinoffs have provided alien or other pseudoscientific explanations for vampires, werewolves, witches, ghosts, faeries, changelings, demons, daemons, mummies, Frankenstein, UFOs, astral projections, mental telepathy, ESP, clairvoyance, spirit photography, telekinetic movement, full-trance mediums, the Loch Ness Monster and the theory of Atlantis. Three times, in the case of Atlantis.
Any character type from The World Of Darkness games can be explained by putting “Space” in front of it...
So choose a story, look at the basics, decide what’s right and wrong in the legends, figure out the being’s agenda or the source of the manifestation and work out how Skywatch cross paths with it and how (and if) they can stop it.
For one example, the Leanhaun Sidhe.
Disappearances do not normally fall under Skywatch’s remit, but when someone disappears mid phone call on a road running past an ancient stone circle... where there are reports of disappearances every generation for over two hundred years...