Amity ~ I really like Amity, I think it's a very pretty and underused virtue name. The fact that it means friendship is a great plus, but I don't pay a whole lot of attention to name meanings. It does remind a bit of Divergent, but that's not a bad association. Some other underused (and maybe a bit out there) virtue names that I like are: Verity, Clarity, Rejoice, Happy, Honor, Eternity, and Remembrance.
Cassia ~ I'm not overly fond of Cassia, either pronounced cash-a or cass-ee-a. I much prefer Cassandra or Cassiopeia, or even just Cassie. Other names with a similar Roman style: Aurelia, Claudia, Fabia, Gaia, Junia, Laelia, Cecilia, Maxima, and Sabina.
Celia ~ I adore Celia. I think it's fresh, snappy, and elegant. I like it as both a stand alone name, and as a nickname for Cecelia/Cecilia. Some other names I think are similar in style or Celia are: Cecily, Margo, Cilla, Audrey, Esther, Mabel, Sylvia/Silvia, Alice, Willa, and Ada.
Eleanor/Eleanora ~ I prefer Eleanor over Eleanora, but both are pretty. I feel like Eleanora/Eleonora is the frillier, girlier version of Eleanor, and it feels a bit more princessy to me. I love all of the nickname potentials from both names; Elle, Ellie, Ella, Nora, etc. I like a few variations of Eleanor as well; Elinor, Leonor(e), Eliana, Ellen, and Elena. Other names with a classic, elegant, or princessy style are: Alice, Clara, Penelope, Catherine/Katherine, Amelia, Beatrice, Charlotte, Florence, Josephine, Arabella, Anastasia, Matilda, Pearl, Annabel, and Susanna(h).
Greta ~ I'm not a fan of Greta, it just feels clunky and stuck in the past to me. Some names with a similar origin/style/sound: Adela, Enid, Greer, Louisa, Maeva, and Theodora.
Gwendolyn (Guinevere is on my list as a middle) ~ I love Gwendolyn/Gwendoline/Gwendolen. It has a personal meaning for me, and it will be the middle name of my first daughter (if I have one). Gwen is a strong, spunky nickname, and I can imagine a little Gwen having lots to say. Guinevere is also beautiful and on my list, but I agree that I would only use it as a middle name. Other names with similar sass and strength to Gwendolyn: Beatrix, Daphne, Genevieve, Heather, Naomi, Philippa, Rosalind, Veronica, and Ruth.
Marian ~ I've been warming up to Marian, slowly but surely. She still has an older feel to me, especially since I know a few older ladies with the name, but I can see a little girl wearing it well. Other names that I feel are similar: Elaine, Winifred, Joan, Joyce, Kathleen, Sheila, Ann(e), Barbara, and Dorothy.
Octavia ~ I like the sound of Octavia, but I'm not sure I can imagine it on a person. It definitely has a strong sound, and I like it as a less popular alternative to Olivia. Similar names: Lucina, Minerva, Lavinia, Prudence, Honoria, Constantina, Raphaela, Livia, and Eugenia.
Ottoline ~ So so pretty. Ottoline is a recent love of mine, but I simply adore it. Personally I would use it as a middle name, but with the nickname Ottie or Otter (<3) I think it would be so lovely. Some more names similar to Ottoline: Thomasine/Thomasina, Jamesina, Bernadette, Albertine, Celestine, Henrietta, Francesca, and Clementine.
Ansel ~ I'm not a fan of Ansel, it reminds me a little bit of Hansel said in a French accent. Some similar names: Adlai, Auberon, Bertram, Eames, Joaquin, Johan, and Willem.
Caspian (I have Cassius and Castor tentatively on my list) ~ I love Caspian, it's so soft and handsome, and I have a bit of a thing for softer sounding boy names. I like Castor as well, but not Cassius (or Cassian). Some softer boy names: Asher, Isaac, Henry, Milo/Miles, Jasper, Colin, Felix, Seth, Grey, Leo, Asa, Noah, and Aubrey.
Eoghan (pronounced like Owen) ~ I like Owen, and I like this spelling of it as well. I think you'd have trouble getting the proper pronunciation or spelling most of the time though, and people who read it will be more likely to say og-an, eg-an, ee-og-an, or something similar. Similar names: Ciaran, Euan, Finian, Oisin, Rory, Tadhg, Declan, and Conall.
Gideon ~ Gideon has all the things that I like in a name, but it just doesn't do it for me and I don't know why. It's very handsome, and it feels very modern as well, despite being a biblical name. Some similar names: Adam, Amos, Abel, Abraham/Abram, Ezekiel, Elias, Gabriel, Nathaniel, Silas, Simon, Josiah, and Phineas.
Lorcan ~ I like Lorcan, but it's a GP for me. It has a lovely Irish origin, which seems to be becoming more popular right now. Similar names: Crispin, Angus, Fergus, Connor, Duncan, Roscoe, Eben, and Lewis.
Lysander (bit of a GP) ~ I love Lysander, and I'd use it as a middle name for sure. I'd have it as a first name, but I love using nicknames, and I'm not fond of Sander or Andy. Some similar names: Bartholomew, Lucian, Balthazar, Sylvan, Peregrine, Cosmo, Valerian, Solomon, Thaddeus, and Leander.
Percival/Percy ~ I do prefer just Percy over Percival, but both are handsome names. Percival is lovely, it's handsome and strong, but I do like softer boys names, and the v in Percival puts me off a little bit. Similar names to both Percival and Percy: Archibald, Fitzwilliam, Hamish, Montague, Wallace, Wolfgang, Alastair, Bastian, Edmund, Crispin, Vincent, and Zachary.
Theodore ~ I love love love Theodore. It's so handsome, soft, and strong at the same time, and I love the nicknames Theo and Teddy. Other names similar to Theodore: Sebastian, Frederick, Edward, William, Gabriel, Alexander, Charles, Louis, and Oscar.
I really love a lot of your names, and I feel like we have similar styles. My favourite names from your list would have to be Eleanor, Gwendolyn, and Ottoline for the girls, and Caspian, Percy, and Theodore from the boys. I hope this helped you a little bit, and that my rambling wasn't too distracting!