DC Sirens could be a cool product. However, the DC universe has not confirmed Batman's allies, so it makes it harder.
Personally, if I was to do this movie, I would do it as a movie taking place after Suicide Squad and treat it like a sequel. Harley Quinn has a falling out with the Joker. And in a solo bank heist gets caught up with Poison Ivy (Like the TV show episode) Cat Woman gets involved somehow and is kinda a mole/double agent character working with Nightwing or the Birds of Prey.
Boom you introduce either A)Nightwing into the universe or B)The Birds of Prey Oracle, Black Canary, Huntress, Cat Woman, I dunno on other character (Let's pull Katana from Suicide Squad into the mix for more carryover) Make it a fun comedic vibe, but more lighthearted than even Suicide Squad.