It's actually a tough criteria, with so many properties being adapted for movies or TV nowadays. I can think of a couple that could benefit from the Deadpool treatment (that is, base it on the source rather than the adaptation).
The Hitchhiker's Guide to the Galaxy
The Dark Tower. Maybe for this one they could develop an original storyline where you're a time traveler whose mission is to erase that godawful movie from existence.
How about Terry Pratchett's Discworld series? No shortage of source material there.