Sequels to films that never were

[Alex]

Well, if starting in the middle of something qualifies, then Indiana Jones (what adventures/exploits led to that start) and Saving Private Ryan (a lot of unfilmed stuff leading to the landing of LSTs at Norandy) and Pirates of the Caribbean: Curse of the Black Pearl (all of the unseen/unexplained goings on with Jack Black that got him onto a sinking dinghy).