1. Lethal Weapon 5
This one is the saddest of all because who doesn't love Lethal Weapon? It was the movie that made many fall in love with Gibson and the buddy cop genre in the first place. The franchise is extremely profitable and for a while a fifth film seemed to be chugging along, but it fell apart for completely different reasons. According to some reports, the studio wanted original screenwriter Shane Black to direct from a script he wrote. Gibson disagreed and wanted longtime collaborator Richard Donner directing based on a story Donner developed. With no agreement made, the movie fell apart. And now with Gibson's public image worse than ever and him and Glover not getting any younger, it seemed Lethal Weapon 5 will never happen. Perhaps things would have been different for Gibson had he let Shane Black take the reigns of the franchise. After all, the guy ended up directing the blockbuster Iron Man 3 this year... Still, the film would've worked even with Richard Donner behind the camera (producer Joel Silver didn't seem to think so). Donner hasn't directed a movie since 16 Blocks (2006), so it would be nice to see another film from him even if it does star Mel Gibson.