I honestly think Diana just doesn't have time to be the leader, and, for that matter, neither do Bruce or Clark. The leader of the Justice League shouldn't have a secret identity (and yeah, neither should Diana, but she should have diplomatic duties to attend to, what with being the Themiscyran princess-ambassador we all wish she could go back to being) or really much in the way of outside obligations, which is why the job so often falls to J'onn (as well it should). Diana could conceivably foist some of those on Donna or Hippolyta, but that would be difficult and out of character, not to mention rude. Dinah works in the interim because she doesn't have much in the way of secret identities and has plenty of people to handle her responsibilities at home (perhaps Ollie could watch her daughter, if she hadn't been SHIPPED OFF TO A MONASTERY), but it's not what she particularly wants to do. So really, the only thing we can do is wait for J'onn to come back. It's just not a Justice League without a Manhunter from Mars.