So this is for the movies in which actors who were attached either dropped out of the movie, and were replaced with another actor, or were in negotiations and then decided to walk away from it. But you think the movie would have been more interesting with them in the it. Off the top of my head I would say:

-Patrick Swayze was supposed to star in Predator 2 originally before dropping out due to being injured on Road House. Arnold also passed on Predator 2 in both the lead and Gary Busey's role.

-Patrick Swayze was going to play Kurt Russell's role in Tango and Cash.

-Bruce Campbell in The Phantom and Darkman.

-Van Damme and Segal were supposed to star in Demolition Man but neither wanted to be the bad guy.

-Robin Williams as The Joker in Batman and The Riddler in Batman Forever.

I can't see either of those guys in the Snipes role for Demolition Man, Snipes hammed that role up perfectly.

Idris Elba as Alex Cross

Although Lee Marvin was solid, I would have liked it if Charles Bronson stayed attached to The Delta Force. Norris and Bronson would have been ultimate.

Emily Blunt as Black Widow.

Adam Sandler as the Bear Jew in Inglourious Basterds

This. I was fine w/ Roth. He played him like I read him in the script. BUT... Sandler is great when you put him to actually act w/ effort.