I love Rebecca, Isabelle and Isabella. Alice is nice but I much prefer the others. Would probably go for Rebecca as there will be so many more of the other two (and as someone who was named Rebecca in the 80s I never thought I'd be able to refer to it as uncommon! 
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You've probably considered all these but, on similar lines, I would think:
Abigail, Annabelle, Amy, Alexandra, Alicia, Bryony, Charlotte, Claire, Cassandra, Emily, Elizabeth, Francesca, Felicity, Grace, Gabriella, Hannah, Harriet, Imogen, Isadora, Jessica, Joanna, Juliet, Katherine, Lucy, Lydia, Madeleine, Matilda, Miranda, Melissa, Nicola, Natasha, Natalie, Olivia, Philippa, Phoebe, Rachel, Rhiannon, Rosemary, Saskia, Sophia, Samantha, Susannah, Thalia, Tamsin, Victoria, Vanessa, Verity, Zara