5. Shrek 5
Despite the fact that the last two Shrek movies weren't all that great, they still made a decent amount of money at the box office, pulling in $789 million dollars and $752 million dollars respectively. Okay, so "decent" might be an understatement. Whichever way you look at it, that's a heck of a lot of dosh.
So although Shrek 4 ("Shrek Ever After") was poised as the last movie in the franchise (posters had "The Final Chapter" stamped all over them), rumours of Shrek 5 began to circulate soon after. And then Eddie Murphy was telling everybody that it was happening, and then it seemed like it was going to happen, and then... nope, Shrek 5 just disappeared into nothing, and fans have heard nothing since.
Granted, if DreamWorks had embarked on Shrek 5 - and more importantly, had they embarked on it and made it as good as the first two movies - it could have easily made a billion dollars. After all, Shrek 3 and Shrek 4 very nearly made it to the billion dollars mark, and they were - all things considered fairly - lackluster affairs. Even now, there's no reason why Shrek 5 still couldn't break box office records.