My recent novella, The Well, works as a standalone story given the little slap-your-face twist at the end. Some of my favorite sci-fi short-story writers employed this strategy on a regular basis: give a story a twist ending that does not really settle the story but does make you think. This approach can annoy, I know. But it also provides a jumping off point for personal imagination. Thank you, Arthur C. Clarke, Gordon R. Dickson, et. al.
That said, The Well begs for more telling, and I intended to expand the story from the beginning. The second as yet untitled novella is nearly done, pending final copy edits and a cover. Oh, yeah - and a title. There is always the pesky nuisance of thinking of an appropriate title, which is often more difficult than it should be.
Anyway, the sequel is coming, and a third installment will wrap up the story. All is Well. Stay. Tuned.