I don't see how a crossover would work well in film. (BTW Luke Cage/Iron Fist wouldn't be a crossover, it would be a comic adaptation - there was a Cage/Fist team comic.)
The biggest problem would be that in order to tell a story, the characters would be slightly altered to follow a plot line. It's hard to explain. Look at Spider-Man... Raimi is taking the character that he has created and forming a story around that character, i.e. what villains best complement Peter's place in life right now, what villains best challenge the character that has been developed so far.
Whereas with a crossover, the main focus for the storyteller would be relaying a reasonable scenario of why these two characters would team up. And in doing so it would be far more easier to mold the character to the story than to mold the story to the character.
I don't want to see Wolverine or Daredevil or any film version of a character do something or act a certain way that would conflict with the established film development of that character.
I wish I could better articulate the point I'm trying to make. Look at George Lazenby when he played James Bond in "On Her Majesty's Secret Service." People had a real problem with Bond breaking down and crying and losing his grip when his wife got killed. Why? Because that's not how the film version of James Bond was expected to act. It was a way the film version of Bond had never acted before.
And I'm afraid that's what a crossover film would do to one of our already established characters.