I know I've mentioned before that I'm a big fan of the "yes, but ..." ending: the main character succeeds in their goal, but the getting of it creates new complications, and the reader is left with the feeling that life goes on. Just as invisible past events lead into the story, invisible future ones flow out of it.
The pat, tidy ending where everything resolves is not a favorite of mine - sometimes in very short stories, but in those tales, there's often not room to introduce extraneous elements in the first place. The demon-summoning sorcerer may be in love with his childhood sweetheart or have gambling debts, but it's not relevant to the plot, so the reader never learns it.
I enjoy creating loose ends in fiction - it makes the end result feel more organic. As long as the main story question is answered, other, supporting questions can sometimes be left dangling ... or have a negative answer, rescuing the story from tooth-ache levels of sweetness.
And these other threads don't even necessarily need an arc. It may be a static element - even something inherent to the world that the character clashes against, but it's not a problem that they can solve. If they tried, well ... that's a whole book in itself, if not a series. (Maybe the NEXT book ... hmm ...)
I think these loose ends contribute to the iceberg effect, the feeling a reader gets that there is a lot more to this world and these people than ever shows up on the page. And maybe - just maybe - it makes the real victory, the struggle the story was, after all, about, that much more satisfying: even if everything else is uncertain, THIS went right. THIS is my success.