Nina - quite like it but wonder if she'd gets lots of 'nee nah' ambulance type pronunciation/jokes.
Greta - it's ok. I would assume German/Austrian heritage but that may well be a totally false association.
Nola - different. Quite like it.
Cleo - like it. I think it's funky and modern.
Janie - Not keen. And I would use Jane or something as official name.
Joni - Like it but only as a nn. I'd called her Joanna.
My choice would be Joni if full name was Joanna and Cleo if not.
On similar lines, how about:
Astrid, Aphra, Bronte, Brodie, Coco, Clea, Carys, Delphi, Devon, Esme, Elsie, Edie, Gretel, Gina, Halle, Ivy, Iris, Iona, Jade, Joelle, Katy, Kiki, Lola, Lois, Luna, Lyra, Marta, Marnie, Mala, Nyah, Nia, Nova, Nora, Nancy, Orla, Rosa, Romy, Sorrel, Tara, Tessa, Willa