I've been told that the reason can't be just because she's awesome. That's some sort of 'opinion' and when listing reasons why someone or something should be a part of something significant, you need to supply facts and reason. Well, I have absolutely no problem with that. In fact, it'll probably be one of the easiest things I've ever done.
She's a Total Badass
Fearless, powerful, and a worthy foe of any adversary, even Wonder Woman and Superman, Hawkgirl has demonstrated she needs no excessive amount of super power to get business properly taken care of.
The DCCU Needs More Female Power Than Just Wonder Woman Can Deliver
The burden of female empowerment certainly doesn't fall solely on Wonder Woman's shoulders in the comics, animated films, or TV shows, why should that be any different for the big screen? I do applaud DC for beating Marvel to the punch when it comes to granting a female superhero their own film, something the MCU should have gotten the ball rolling on a long, long time ago. In the beloved Cartoon Network series, Justice League and its sequel series, Justice League Unlimited, (both are on Netflix if you haven't seen them. They're fantastic) Hawkgirl and Wonder Woman are the only senior female members of the League and because so, they are not as often called on as their male counterparts. However, when Wonder Woman's adopted sister strikes, developing a disease that affects only men, it becomes the burden of these two heroes. along with female firefighters, police, and doctors, to save their friends and the world.
Her Emotional Connection to Solomon Grundy
The pair formed an unlikely bond when heroes and villains had to team up to save the world from certain doom. A task that would claim the life of the former mobster turned gargantuan zombie. When he is resurrected by a couple of naive nerds attempting to get revenge on their bullies, the League calls upon Shayera to end his suffering, a task that takes a terrible toll on her already broken heart.
She Turns on Her Own People to Save Earth
The Thanagarians come to the aid of Earth when they are attacked by an unknown race of deadly aliens, or so it's made to seem. Initially on their side when the Thanagarians invade Earth and assume control, Shayera comes to her senses and aids the Justice League in their battle against the desolation of the human race. By then, the majority of the League has already made up their minds that Shayera is a traitor and can no longer be trusted. After the League sends the Thanagarians packing, they convene at Wayne Manor to decide whether or not Hawkgirl should be allowed to remain a part of the team. Before they can decide, she abruptly resigns and bids them all farewell. Which brings me to my final point...
Her Romantic Relationship with John Stewart/Green Lantern
With the news that Tyrese Gibson is super interested in portraying the first African-American hero of DC Comics, (Please DC? Please?) the addition of Shayera Hol would only make that much more sense. The two have the best chemistry of any two characters within the DC Universe. Their love story, though complicated, would make for some excellent side plots in the ever-expanding DC Cinematic Universe.
Think about it, WB, okay? I bet you could even get Noomi Rapace to play her.