Anything: Series, video game, book, food, old toy from childhood, service, etc. Also, any series which you wish did not end, etc.
The title has two questions and this one only fits one of them.
Wish wasn’t cancelled, yes!
Wish they brought it back? No, it was a long time ago, and they made Serenity to close off the story and tie things up. It’s likely that bringing it back would do more harm than good.