I kinda picture them as the Black Widow and Hawkeye of the film (minus the romance, of course). With everyone else being newly introduced and unsettled with each other, they'd at least be the one constant.
Which, I think would be the most interesting as their personalities would be the most immature and roguish. That would say something. Here the ever mature Batman and Superman would be struggling to adjust to each other's divergent personalities, yet two total, yet kinda immature, opposites in GL and Flash would be best of friends.
Additionally, I'd make Wonder Woman the oldest of the group for similar reasons. Batman and Superman have such a gravitas about them, and in a lot of ways, a certain level of arrogance. Wonder Woman not only being wise beyond her years, but also a WW2 legend would, again, through Superman and Batman's idea of the status quo for a loop. Suddenly they'd be the ones who are intimidated, who are young and unproven compared to her.
Then, throughout the story, you have all of those different personalities learn from each other, and adapt to each other.