Hi,
I'm from a European non-English speaking country and we have all sort of names here: French, Scandinavien, German, etc. So maybe I can shed a light. or myabe I'm totally wrong. ;-)
Names that sound a bit like Isla:
Isa, Indra, Ayla (pronounced with 'aah', not 'ay' like in Kay), Elya, Ilya, Arla.
Other ones:
Nola, Nyla, Nilda, Nessa, Nissa, Noa, Ebba, O(o)na, Lola, Luna, Luca, Mona, Ella, Lissa (yes, dubbel 's' so shorter 'i' dan Lisa), Tessa, Mina, Gilda (maybe too simular to Gaia), Fia, Sophie/Sofia, Liva, Enya, Fenna, Mena, Assiya, Mila, Nayla/Naïla, Nikka, Stella, Lina, Lia, Layla, Iva, Kayra, Ina, Mischa, Lexa, Yara, Lila(h), Inya, Lilja, Tia, Nala, Cara, Orla, Tia, Nena, Nuria, Rania, Maya, Mia, Zina, Sienna, Zita, Livia, Kyra, Ranya, Mara, Maura, Myra, Ani, Estelle.
You also seem to like mythological names. Here are a few: Thalia, Clio (but might be a bit confusing as it is pronounced the same way as Cleo in English), Lio (same thing as the pronounciation is like Leo in english), Shiva, Freya, Daphne, Iris.
Similar to Juno: Juna/Yuna.
Astrid is a Scandinavian name, so here are a few more: Malin, Merit, Marit, Bryndis, Ilva, Stina, Lotta.