The problem I have with Suicide Squad is needless excess. In the theatrical version of the story, Captain Boomerang and Killer Croc serve almost zero purpose. In the words of Nick Mason on the Weekly Planet, they're given "odd jobs" to do just so they feel like part of the team. But like I said, they could be cut out and there's no real effect on the plot. Someone else throws the bomb. That's it. Captain Boomerang only throws three boomerangs the whole movie, and they give him that "camera boomerang" gimmick just so he feels important (they were upset because they're Australian
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Joker's "present day" story, in the theatrical version, serves almost no purpose. If you cut it out entirely, then there's just two effects:
1. Amanda Waller gets on the second helicopter "first"
2. You either have to explain why Harley is separated from the group after the helicopter crash or just cut out their reunion (either of which would work seamlessly).
Other than that, there's just some text messages. Joker's role could've simply been in the flashbacks (and if you want a final shot, him breaking her out of prison) and it still works. You could even have him be "dead" from Batman instead of the helicopter crash, and her surprise that he's alive still works.
If you can cut that much of a character from a film and create almost no plot holes, then the character doesn't need to be there. And I thought Leto's Joker was boring enough that he could've been cut, and my enjoyment of the movie would've been negligibly affected.
So I'd be counting on deleted scenes to justify the existence of these characters. If I'm supposed to care about Killer Croc, Captain Boomerang, the Joker, or Katana, it's gotta be in deleted scenes. Otherwise, those characters didn't need to be in the movie, and the "extended cut" should be versions where they're edited out. Because, yeah, teams need redshirts, but Slipknot has more impact on the story than Joker/Boomerang/Croc. And he's barely introduced, and he's in the movie for two minutes.
That's a problem, IMO.
As a side exercise, try to take out someone like Hawkeye or Fury from the Avengers and see what plot holes arise. Even characters like Maria Hill and Dr. Selvig have crucial elements that cannot be eliminated. Same with Guardians. If you want to talk non-comic book movies, even something like Star Trek has a crucial scene for every main character. Not just participating in a large action sequence, something they're actually contributing to the story.
That's the problem. Imagine a Justice League movie where Cyborg does almost nothing for the entire plot and can be edited out without causing any plot holes. It'd be a huge problem.