Anything: Series, video game, book, food, old toy from childhood, service, etc. Also, any series which you wish did not end, etc.
As long as we’re on cancelled sci-fi shows, Babylon 5 spinoffs. The entire setting is more or less dead and it hurts.
(Yes, I know about The Road Home. It’s not the same.)
Problem with that is that almost everyone who was interesting is dead. Actor wise.