at least in the west, when you sign a contract to create something, sequels are usually what's called "optioned". so for example: you go on and create the thing, then it gets cancelled. the publishing company has exclusive rights to the options to make a sequel, should they want to (why it's called an "option" - they're not obligated to make it). if some other publisher wanted to continue the story, even if the creator wants to also, they couldn't without first acquiring the rights to those options from the original publisher (usually for money). And it doesn't usually work in reverse, whereby the artist approaches the publisher and says "let's continue my work from years ago", unless it's some superstar artist maybe. The publishers hold most of the power for these things. It's possible that there's a ticking time bomb on those rights, where the options will revert to the creator at some point in the future, but that's not really knowable.