I think there are 2 types of underused - known but rare and currently out of fashion:
Always underused ideas:
April, Anastasia, Antonia, Asha, Asia, Aubrey, Amberley, Anika, Bronte, Brodie, Carmen, Cressida, Cassandra, Cordelia, Clarissa, Cecily, Catriona, Erica, Esther, Effie, Fallon, Faith, Fenella, Fabia, Giselle, Henrietta, Harmony, Iona, Juliet, Jocelyn, Jacinta, Jessamy, Joanna, Karina, Leonie, Larissa, Lavinia, Lorelai, Lois, Mara, Mala, Melody, Miriam, Naomi, Nadia, Ophelia, Octavia, Paloma, Pandora, Saskia, Serena, Thalia, Tamsin, Tanya, Verity, Vanessa, Xanthe, Zia
Currently out of fashion ideas:
Alison, Amy, Anne, Anita, Aileen, Belinda, Bridget, Beverly, Bryony, Brenda, Barbara, Claire, Coleen, Cathleen, Donna, Denise, Deborah, Dawn, Diana, Diane, Demi, Eileen, Elaine, Fiona, Freda, Gloria, Hannah, Hazel, Hilda, Hilary, Irene, Julie, Judith, Justine, Jennifer, Joanne, Karen, Kimberly, Kayleigh, Kylie, Kelly, Kerry, Lisa, Lyndsey, Louise, Melissa, Melanie, Marie, Natasha, Pamela, Queenie, Ruth, Rita, Roxanne, Rachel, Sarah, Susan, Suzanne, Sally, Stacy, Sharon,Tiffany, Tracy, Ursula, Vivien, Valerie, Wendy, Wanda, Yvonne, Yvette, Zoe
Will do boys later