I mean you can basically do the classic costume, while tweaking a few details here and there to make it more "cinematic." They've done it with Wonder Woman, Batman, Superman, Spider-Man, Deadpool, Iron Man, Captain America, Katana, Etc. So I don't think that Harley is beyond that.
Also, she's meant to look like a clown. Her name is HARLEY QUINN for goodness sake. Aside from he face-paint (and even that's stretching it), there's nothing about her current DCEU look that screams "clown."
Wait! She wears red lipstick! That's clowny enough! /s
They couldn't even do the correct diamond patterns. I don't see why, if they were so determined to go for the tattooed look, she couldn't have the correct pattern on her thigh. Three diamonds. That's it. That's all they had to do. She even had it in her concept art ffs.
). It's only saving grace was that it was launched as part of the Batman Reborn initiative spearheaded by Grant Morrison.