13. World War Z 2
Despite enormous production problems, World War Z was a stonking box office hit back in 2013, so it's pretty surprising that a sequel wasn't fast-tracked for a 2015 or 2016 release.
Attempts were made, with J.A. Bayona (Jurassic World: Fallen Kingdom) originally tapped to helm for a summer 2017 release, but he bailed for greener pastures, and in mid-2016 it was rumoured that David Fincher was in talks to helm.
Fincher has since been tacitly confirmed as director, though his commitments to the Netflix series Mindhunter have pushed production back, and he's insisted that he won't direct the film if the script isn't right.
It's hard to imagine Fincher ever being happy with the script for a movie like this, honestly, and the longer the project lingers around, the less interested audiences are likely to be in a follow-up.
Considering the huge costs involved in a sequel, Paramount may simply decide to cut their losses if Fincher bails and close to a decade passes without a script and director being nailed down.