Looking at the source material, there are a lot of obvious and not so obvious heroes and villains who deserve to either make their big screen debuts or return to theaters in this movie, and what you'll find here are the eight likeliest candidates. While a writer is currently penning the screenplay, we actually know very little about what to expect from this Harley Quinn movie and it could be some time before we hear anything official. Even so, don't be surprised to see this lot being cast pretty soon...
8. Poison Ivy
The last time we saw Poison Ivy in action (not including the younger version from Gotham) was in Batman and Robin, so this Batman villain's return to the big screen is long overdue. Ivy has emerged has something of an anti-hero in recent years, with her quest to save all plant life more relevant than ever due to what's happening in the real world.
Throw in the fact that she's also one of Harley Quinn's best friends and a member of both the Gotham City Sirens and Birds of Prey, and an appearance from her here is pretty much guaranteed. Visually striking and delightfully insane, Poison Ivy has a ton of potential and could easily cameo in The Batman before showing up in this movie.
7. Huntress
There are a couple of different versions of Huntress, one of whom is the daughter of Bruce Wayne and Selina Kyle from another Earth. However, the Helena worth focusing on in this movie would be Helena Bertinelli, a young woman born into one of Gotham's mafia families who became a violent vigilante dead set on destroying those who killed her family.
A member of the Birds of Prey for a time, she's a little more brutal in her methods than some of the more heroic members of the team, but Warner Bros. could also borrow from the New 52 version who remains Sicilian but is actually secret agent for Spyral. Huntress being sent in to infiltrate and spy on this team could work.
6. Manhunter
Kate Spencer made a brief appearance in Arrow a couple of seasons back, but we never got to see her suited up as Manhunter. Similar to Daredevil in many ways, she's a lawyer and former district attorney who took on this identity in order to right the wrongs she believed the judicial system could or would not.
However, in terms of how Manhunter deals with crimnals, she's a lot closer to The Punisher, so throwing her into the mix here as someone who can relate to both the more heroic and villainous members of the Birds of Prey makes sense. Despite being a relatively obscure character, Manhunter has quite a bit of history with this team, so don't be surprised to see her show up.
5. Big Barda
If rumours about Steppenwolf being the main villain of Justice League before Darkseid takes centre stage in the sequel are accurate, then chances are that the DC Films Universe is going to spend quite a bit of time exploring Apokalips. If that's the case, introducing Big Barda (who once served as the leader of Darkseid's personal guard, the Female Furies) is a must, especially if the story of her finding love with Mr. Miracle and fleeing that life is also squeezed in somewhere.
Assuming this Harley Quinn led spinoff is still a few years away, Barda - an established character by then - being added to the mix would be smart as it gives this team some muscle and an outsider whose presence shakes up the team dynamic.
4. Batgirl
We're going to have to wait until the release of the Ultimate Cut of Batman v Superman: Dawn of Justice in order to find out whether Jena Malone is playing Barbara Gordon, Batgirl, Oracle, or someone else entirely, but a version of Jim Gordon's daughter factoring into this spinoff is a must.
Oracle certainly has her advantages, but it would arguably be far more satisfying to see Batgirl on the big screen, especially after her disappointing portrayal in Batman and Robin. Her serving as the leader of this team but constantly butting heads with Harley Quinn would make for fun viewing, and playing on the history between them and The Joker has a ton of great dramatic potential which is well worth exploring.
3. Katana
Assuming she makes it out of Suicide Squad alive, Katana showing up in this movie seems pretty certain. Despite being Rick Flagg's bodyguard and seemingly on the right side of the law, this deranged anti-hero (remember she communicates with the souls her sword contains) could easily be drafted in to whatever Harley is up to even if she's only sent there to keep an eye on the villain.
It also seems likely that Warner Bros. will want to include at least one more character audiences will be familiar with, especially if Suicide Squad ends up being the hit they're hoping for this August. Considering the fact that Katana has plenty of ties to the Birds of Prey in the comics, it would be a shock for her not to appear here.
2. Black Canary
There have been some pretty baffling rumours about how Black Canary will factor into Justice League, but her having some sort of role in the movie is something we've known for a while now. It's also no doubt why Katie Cassidy's version of the character was dropped from Arrow recently, something we've seen happen quite a few times now with DC Comics characters from the small screen set to make appearances on the big one.
Black Canary is an iconic part of the Birds of Prey so must be included here, and introducing the character on a stage as grand as Justice League is a perfect way to ensure that moviegoers are interested in following her story into whatever this spinoff ends up being.
1. Catwoman
With any luck, Catwoman is going to appear in The Batman (hopefully played by Gina Carano), but a role for this thief turned anti-hero in whatever this movie ends up being is essential. Even if it's just a cameo, Selina Kyle butting heads with the likes of Harley Quinn and Batgirl is extremely appealing and she's well known enough to bring in casual moviegoers who may not be that familiar with other members of this team.
While Harley is no doubt going to really take off after Suicide Squad, Catwoman is definitely the kind of character this movie could also be heavily be promoted around, and as good as some of the previous movie versions of Selina have been, it still feels like we've yet to see the definitive version from the comics.
Do you want to see these characters in the Harley Quinn/Birds of Prey movie? Have we missed any of your favourites? As always, let us know your thoughts in the usual place.