I've been thinking about it and I think, aside from the editing, one of the huge underlying problems with this movie is that the plot isn't personal enough. It's too large-scale and thus fails to capture any real tension. How many god damn times are we gonna get the "oh no the magic butthole portal in the sky" schtick with superhero movies?
What made Ostrander's original run on the comic was that the Squad was dealing with tense, politically-messy threats that a team of soldiers or heroes couldn't.
In my opinion, a plot centered around the Joker would have been better, rather than introduce another complicated threat that would take up valuable time.
Basically, Wallet catches wind of some plot by the Joker (maybe mass-bombings or something like that) and sends in the squad to kill him before he can enact this.
This gives you more of a reason to explore Harley and Joker's relationship (which should have been abusive, but wasn't in this movie) and gives her a more tragic arc, by having her wanting to get back at the Joker for abandoning her, but upon meeting him again, returns to his side. Plus, you can bring Batman into the proceedings in a natural way. He doesn't believe in killing(anymore), and would definitely try to throw a wrench in the Squad's mission. This allows him to cross paths with Deadshot again, and to have a more direct confrontation with Waller about the Squad's actions. And we should have gotten a flashback to Jason Todd's murder by this point.
t:
. Wonder Woman vs Harley Quinn 
