These days, original ideas seem sort of hard to come by. Don’t believe me? Go see a movie, but be sure you make it in time for the previews. Now, notice how nearly every movie you see is cashing in on an existing franchise? Sequels, prequels, adaptations and reboots account for so many of the blockbusters these days, and I don’t blame the studios for cashing in on properties they can depend on for a payday.
Adaptations are a necessary risk. Sure, there’s a chance you’ll end up making a Super Mario Bros or a Daredevil, but the other side of that coin is a Jaws or a Jurassic Park. I suppose a good rule of thumb would be to try and stick to novels while avoiding video games.