Well,
Atalanta was really awesome and just about as superhero as a Greek goddess could get (aka, super awesome until eventually some guy outsmarted her). I would say
Artemis, but she's just the Greek form of
Diana, so that wouldn't work out too well. Oh! Oh oh oh! Wait! How could I have forgotten
Athena?!
Athene (with the e at the end, not the a) is one of my favorite names, though the a works as well. And
Athena was obviously quite strong and heroic, what with being the somewhat war-ready goddess of wisdom.
Heck, they could even use her full name, if they wanted:
Pallas Athena.
Pallas,
Athena or
Athene are all up for grabs there, as is
Minerva, her
Roman name. And then, of course, there is
Nike, the Greek goddess of victory, but I'm not so sure they want to attach their daughter to a sporting brand. There is always the
Roman equivalent,
Victoria, but do they want something so discreetly god-like? Actually,
Victoria may work. It goes well stylistically and thematically with
Diana. I don't think anything I mentioned had a Z sound. Oh well. I hope I was helpful nonetheless!