(or "Like '100 Bullets', But Without The Bullets")
The problem with creating an open-ended series from H.G. Wells' classic novel, 'The Invisible Man', is fairly obvious when you read it; not to spoil the big surprise, but the title character winds up dead by the end of the story. This makes it a little bit tricky to continue telling tales about him (a common theme among Universal's horror films, as we've seen in the past. Dracula, Frankenstein, the Wolf Man, the Mummy and the Creature from the Black Lagoon all met similar fates.)
But, as with those other monsters, that didn't stop Universal from deciding to make a sequel to a profitable film. It fell to Lester Cole, Curt Siodmak, and Joe May to find a way to turn the one-off story into a storytelling engine, and they found a doozy. Sure, Jack Griffin, the inventor of the invisibility formula, might have gone mad from the side effects of his creation. Yes, he was shot and killed in order to end his reign of terror. But that's the end of Griffin, not the end of the formula.
The invisibility formula becomes the new center of the sequels that followed the continuity of the original film (which wasn't all of them. "The Invisible Woman" is a screwball comedy with a nutty professor, and "The Invisible Man's Revenge" follows a different scientist with his own formula, which he tests on someone who was pretty insane to start with.) Griffin's brother, Frank, duplicates his brother's research...but runs into the same stumbling block his brother did. There's no way to reverse the formula, and no way to cure the side effect of progressive megalomania. But for a few people who have access to the formula, the power is there...as is the price.
So the question in the sequels becomes, "What would drive someone to use such a formula?" In "The Invisible Man Returns", it's a question of necessity; the main character has been sentenced to hang for a murder he didn't commit, and has to use the formula to clear his name and find the real killer (while scientist Frank Griffin frantically searches for a cure.) In "Invisible Agent", it's an issue of patriotism; Griffin's grandson (the movie seems confused on which Griffin it is--they suggest Jack, but Frank makes more sense) uses the formula to become the ultimate Allied spy, able to walk through the streets of Berlin and steal vital secrets right out from under the Nazis...until his paranoia turns him against his German contacts. It's worth mentioning that both movies do invent a cure, in the form of massive blood transfusions--when the character you're following is more sympathetic than the original glory-hungry Jack Griffin, it's nice to be able to give them a happy ending.
Universal's sequels trailed off after that, but the "invisibility formula" MacGuffin can easily provide more seeds for stories in the hands of an intelligent writer. Giving a power that carries a price to a desperate man or woman isn't just a good way to start a story, it's a resonant one; we've all felt the touch of obsession and its dangers to some degree or another, and whether the protagonist manages to pull back from the brink of madness or falls into its depths, their struggle reminds us of the risks of giving everything up for a single cause.