Movies that teased sequels that never happened

In today's world of mega-franchises and shared cinematic universes, it's easy to forget what a big deal a sequel to a popular film used to be. In the early days of the blockbuster, a sequel seemed like a special gift from the filmmakers to the audience, an acknowledgement that they knew we liked their movie, and they thought that was cool, and wanted to give us more.

As sequels started to seem more like a given than a gift, some movies began setting them up right there in the first film, promising even greater adventures with the characters we'd just come to know. But sometimes, for a variety of reasons, this didn't always work out—and audiences were left hanging, awaiting a next chapter that was never to be.

Warning: spoilers abound.